Carrier-suppressed optical pulse train generating device and method

ABSTRACT

The device is structured to have a first electric modulation signal generator, a second electric modulation signal generator, a two-mode beat light source and an optical intensity modulator. The first electric modulation signal generator generates and outputs a first electric modulation signal. The second electric modulation signal generator generates and outputs a second electric modulation signal of a same frequency as the first electric modulation signal and to which a phase difference of δ radians is provided (δ is a real number satisfying 0≦δ≦π). The two-mode beat light source is driven by the first electric modulation signal, and generates and outputs two-mode beat light. The two-mode beat light is inputted to the optical intensity modulator, and the optical intensity modulator generates and outputs a CS optical pulse train. Light transmittance of the optical intensity modulator is modulated by the second electric modulation signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority under 35 USC 119 from Japanese PatentApplication No. 2008-004435, the disclosure of which is incorporated byreference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a CS (Carrier-Suppressed) optical pulsetrain generating device for generating an optical pulse signal inaccordance with carrier-suppressed RZ (Return to Zero) format opticalintensity modulation or optical phase modulation, and to a CS opticalpulse train generating method using this device.

2. Description of the Related Art

The capacities and long-distance capabilities of transmission of opticalcommunications networks are increasing. There have been proposed varioustypes of formats of the optical signals used in-optical communicationssystems that structure optical communications networks. Among these,several are being put into practical use. A representative opticalsignal format that is being put into practical use is the opticalintensity modulation format that expresses a binary digital signal bythe strength of the optical intensity. There are two main types ofoptical intensity modulation formats, which are an NRZ (Non Return toZero) format in which the optical intensity is maintained during acontinuous “1” signal, and an RZ format in which the optical intensitybecomes zero once during a continuous “1” signal.

An RZ format optical signal is generated by an optical intensitymodulator optical-intensity-modulating the individual optical pulsesthat structure an optical pulse train in the optical pulse train that islined-up orderly at uniform intervals on the time axis. The opticalintensity modulation of the individual optical pulses structuring theoptical pulse train is the generating of a binary digital signal byselectively cutting-off or transmitting-through the optical pulses thatstructure the optical pulse train. In order to generate an RZ formatoptical signal, the optical pulse train is needed in advance, and alight source that generates the optical pulse train is needed.

As described above, an RZ format optical signal is a binary digitalsignal obtained by optical-intensity-modulating an optical pulse trainthat is lined-up orderly at uniform intervals on the time axis.Accordingly, “optical pulse signal” and “optical pulse train” have thefollowing meanings hereinafter. “Optical pulse signal” is used toindicate a row of optical pulses serving as a binary digital signal thatis obtained by optical-intensity-modulating an optical pulse train thatis lined-up orderly at uniform intervals on the time axis. On the otherhand, “optical pulse train” is used to indicate the aggregate of opticalpulses that are lined-up without deficiencies, orderly at uniformintervals on the time axis.

The RZ format is a format in which the optical intensity becomes zeroonce even during a continuous “1” signal. Accordingly, generally, thefrequency band of the light that serves as an electric field of light iswide as compared with the NRZ format.

In an RZ format optical pulse signal, the optical pulses expressing thebits that mean “1” always exist individually on the time axis.Accordingly, the optical pulse signal is structured as an aggregation ofoptical pulses whose full width at half maximum (FWHM) are narrow. Onthe other hand, an NRZ format optical pulse signal is structured asoptical pulses that, when bits meaning “1” appear in continuation, havea continuous wide width during the time that the “1” continues.Therefore, the full width at half maximum of the optical pulsesstructuring an NRZ format optical pulse signal are, on average, widerthan the full width at half maximum of the optical pulses structuring anRZ format optical pulse signal.

Accordingly, the frequency band that an RZ format optical pulse signaloccupies (hereinafter also called “frequency spectral band” uponoccasion) is wider than the frequency spectral band occupied by an NRZformat optical pulse signal. In the following description, there arecases in which simply the term “spectrum” is used in cases in whichthere are no need to differentiate between whether it is a spectrumexpressed by frequency or a spectrum expressed by wavelength.

When the spectral band is wide, first, due to the group velocitydispersion of the optical fiber that is the signal transmission medium,the effect of waveform distortion, in which the full width at halfmaximum of the optical pulse on the time axis widens, markedly appears,and the transmission length is thereby limited. Secondly, when takinginto consideration the increase in capacity in accordance withwavelength multiplexing systems, in order to suppress crosstalk betweenchannels to which adjacent wavelengths are assigned, the difference inthe wavelengths assigned to adjacent channels must be made to be large.In either case, an optical pulse signal of a wide spectral band is notpreferable from the standpoint of the efficient utilization of thefrequency band by the optical communications network that uses thatoptical pulse signal.

Thus, methods of narrowing the spectral band of an RZ format opticalpulse signal have been proposed. A representative method there among isa method that employs a so-called CS-RZ format that RZ-formats anoptical pulse train whose phase serving as an electric field of light isinverted between optical pulses that are adjacent on the time axis.(See, for example, A. Hirano, Y. Miyamoto, S. Kuwahara, M. Tomizawa, andK. Murata, “A Novel Mode-Splitting Detection Scheme in 43-Gb/s CS- andDCS-RZ Signal Transmission”, IEEE J. Lightwave Technology, vol. 20, No.12, pp. 2029-2034, 2002.) The phase that serves as an electric field oflight being inverted between optical pulses that are adjacent on thetime axis, is synonymous with the phase difference between adjacentoptical pulses being π.

Inverting the phase that serves as an electric field of light betweenoptical pulses that are adjacent on the time axis means that the phaseserving as an electric field of light is not continuous, and a phasejumping portion where the phase of the electric field of light suddenlychanges by π exists between the adjacent optical pulses. Accordingly,the effect of the interference that arises between adjacent opticalpulses becomes the effect of offsetting the amplitudes of one another.On the other hand, when the phase that serves as an electric field oflight between optical pulses that are adjacent on the time axis is thesame phase, the effect of the interference that arises between theseoptical pulses becomes an effect in which the amplitudes thereof areadded together.

In the CS-RZ format, the spectral band can be reduced by about 25% ascompared with the usual RZ format in which the phase that serves as anelectric field of light between optical pulses that are adjacent on thetime axis is the same phase. (Refer to A. Hirano, Y. Miyamoto, S.Kuwahara, M. Tomizawa, and K. Murata, “A Novel Mode-Splitting DetectionScheme in 43-Gb/s CS- and DCS-RZ Signal Transmission”, IEEE J. LightwaveTechnology, vol. 20, No. 12, pp. 2029-2034, 2002.) Therefore, the CS-RZformat has excellent resistance to waveform distortion due to the groupvelocity dispersion of the optical fiber, and excellent frequencyutilization efficiency. Further, in the CS-RZ format, even if the dutyratio of the optical pulse signal is high, waveform distortion due tointerference between optical pulses that are adjacent on the time axisis suppressed more than in the usual RZ format. Therefore, the widths,on the time axis, of the optical pulses structuring the optical pulsesignal can be made to be wider than in the usual RZ format. As a result,the spectral band of the electric field of light can be reduced. Namely,by employing a CS-RZ format optical pulse signal, an opticalcommunications system having an excellent long-distance transfercharacteristic/frequency utilization efficiency can be realized.

Here, the duty ratio of an optical pulse is the ratio of the full widthat half maximum of that optical pulse with respect to the intervalbetween optical pulses that are lined-up adjacent on the time axis (thepulse duration per one bit, also called the “time slot”). Accordingly,the duty ratio being high means that the full width at half maximum ofthe optical pulse is wide with respect to the time slot. Namely, if thetime slot is fixed and the full width at half maximum of the opticalpulse is widened, or if the full width at half maximum of the opticalpulse is fixed and the time slot is narrowed, the duty ratio becomeshigh.

The following four methods have been conventionally proposed as methodsof generating a CS optical pulse train that are needed in order togenerate a CS-RZ format optical pulse signal.

The first method is a method of using a Mach-Zehnder interferometer typeLiNbO3 optical intensity modulator (see, for example, A. Hirano, Y.Miyamoto, S. Kuwahara, M. Tomizawa, and K. Murata, “A NovelMode-Splitting Detection Scheme in 43-Gb/s CS- and DCS-RZ SignalTransmission”, IEEE J. Lightwave Technology, vol. 20, No. 12, pp.2029-2034, 2002.) Hereinafter, the LiNbO3 optical intensity modulatorwill be referred to upon occasion as the LN optical intensity modulator.This method will be described by using, as an example, CS optical pulsetrain generation in which the repetition frequency is 40 GHz. First,continuance wave (CW) light, which is produced from a CW light source,is inputted to the LN optical intensity modulator. Then, the DC biaslevel of the control electric signal supplied to the LN opticalintensity modulator (which is a sine wave in most cases) is set to theminimum voltage value of the light transmittance. Further, if the LNoptical intensity modulator is modulated by an electric modulationsignal, whose repetition frequency is 20 GHz and whose intensityamplitude which is the voltage difference between the maximum and theminimum (the peak-to-peak voltage, also called “Vpp” upon occasionhereinafter) is 2 times the half-wave voltage Vπ, a CS optical pulsetrain of a repetition frequency of 40 GHz is outputted from the LNoptical intensity modulator.

In accordance with the first method, even if the wavelength of the CWlight source is changed, the change in the characteristic of the opticalpulse is small, and therefore, a high-performance, wavelength-variableCS optical pulse train generating light source can be provided. This isbecause the wavelength dependence of the optical intensity modulatingcharacteristic of the LN optical intensity modulator is small. Further,the first method also has the advantage that the repetition frequencycan be changed easily.

The second method is a method using a two-mode oscillation laser. Atwo-mode oscillation laser is a laser in which the longitudinal mode ofthe laser oscillation spectrum is formed from two wavelength components,and ideally, the intensities of these two wavelength components areequal. The light output of a two-mode oscillation laser is a CS opticalpulse train, and the time waveform thereof is a sine wave. Further, therepetition frequency of the CS optical pulse train that is outputtedfrom a two-mode oscillation laser coincides with the difference in theoptical frequencies of the two oscillation longitudinal modes.

The oscillation light of the two-mode oscillation laser is an opticalpulse train of a repetition frequency that is equal to the beatfrequency of the two longitudinal mode components. For this reason, thetwo-mode oscillation laser is called a two-mode beat light source.However, there are cases in which a light source, by which there isobtained output light that is formed from two wavelength componentswhose wavelength spectra have equal intensities, is called a two-modebeat light source regardless of the structure thereof. Thus, a two-modebeat light source that is realized by a single laser element is called atwo-mode oscillation laser. When indicating a general pulse light sourceincluding pulse light sources that are structured by combining plurallaser elements, including this two-mode oscillation laser, the termtwo-mode beat light source is used. Namely, two-mode beat light sourceis a wide concept that includes two-mode oscillation lasers.

As will be described later, among pulse light sources that arestructured by combining plural laser elements, there is known a lightsource of a form in which two semiconductor lasers that oscillate in thelongitudinal single mode are phase-synchronously driven, and the twooutput lights that are outputted from these two semiconductor lasers arecombined and outputted. Further, there is known a light source that isstructured such that two-mode beat light is obtained by extracting onlyadjacent two wavelength components among the longitudinal modecomponents by a wavelength filter, from output light of a mode-lockedsemiconductor laser having numerous longitudinal mode components.

A two-mode laser oscillation method is known that uses a mode-lockedsemiconductor laser with which a chirped grating is integrated, andutilizes the dispersion of the chirped grating (refer to, for example,K. Sato, A. Hirano, and N. Shimizu, “Dual mode operation of mode-lockedsemiconductor lasers for anti-phase pulse generation”, Technical Digestof OFC 2000, paper ThW3-1˜3-3, 2000). For convenience of explanation,here, three longitudinal modes in the vicinity of the Bragg reflectionwavelength of the chirped grating are considered. The frequencies ofthese three longitudinal modes are, from the low frequency side, fm−1,fm, fm+1. By using the dispersion of the chirped grating, the frequencydifference (fm−fm−1) between the (m−1)st order and mth orderlongitudinal modes, and the frequency difference (fm+1−fm) between themth and (m+1)st order longitudinal modes, are values that differ moregreatly the more that frequency pulling-in due to mode-locking operationdoes not arise. Here, m is an integer.

When mode synchronization is caused by providing modulation equal to(fm+1−fm) to the mode-locked semiconductor laser, frequency pulling-indoes not arise at the (m−1)st order mode, and therefore, the laser doesnot mode-lock-operate. Namely, the laser two-mode-oscillates.

The above two-mode oscillation laser is not limited to a mode-lockedsemiconductor laser with which a chirped grating is integrated such asthat disclosed in the aforementioned document. Further, the abovetwo-mode oscillation laser is not limited to a mode-locked semiconductorlaser. The two-mode oscillation laser can be realized by a laser thatintegrates a sampled grating (refer to L. A. Johansson, Zhaoyang Hu, D.J. Blumenthal, L. A. Coldren, Y. A. Akulova, and G. A. Fish, “40-GHzDual-Mode-Locked Widely Tunable Sampled-Grating DBR Laser”, IEEE Photon.Technol. Lett., vol. 17, No. 2, pp. 285-287, 2005), or by aself-pulsating distributed feedback semiconductor laser (refer to C.Bobbert, J. Kreissl, L. Molle, F. Raub, M. Rohde, B. Sartorius, A.Umbach, and G. Jacumeit, “Novel Compact 40 GHz PZ-Pulse-Source based onSelf-Pulsating PhaseCOMB Lasers”, Technical Digest of OFC 2004, paperWL5, 2004). In this case, the structure of the element that includes thediffraction grating formation region of the laser that integrates asampled grating or the self-pulsating distributed feedback semiconductorlaser is optimized so as to realize a two-mode oscillation laser.

The third method is a method using an optical pulse light source and anoptical delay interferometer. This method will be described by using, asan example, a case of generating a CS optical pulse train of arepetition frequency of 40 GHz. First, an optical pulse light source isreadied that generates and outputs a usual optical pulse train in whichoptical phases between optical pulses adjacent at a repetition frequencyof 20 GHz are uniform. Next, this optical pulse train is branched in twoby using an optical branching device or the like. By using a delayoptical system, a time delay of 25 ps is provided to one of the opticalpulse trains that were branched in two, and simultaneously, an opticalphase difference of π is provided. Thereafter, by multiplexing bothoptical pulse trains by using an optical combining device, a CS opticalpulse train of a repetition frequency of 40 GHz is generated.

An optical-fiber-type element can be used in the optical branchingdevice and the optical combining device, and in the delay opticalsystem. Further, a method that combines a half mirror and a spatialoptical system (see H. Murai, M. Kagawa, H. Tsuji, and K. Fujii, “EAModulator-Based Optical Multiplexing/Demultiplexing Techniques for 160Gbit/s OTDM Signal Transmission”, IEICE Trans. Electron., vol. E88-C,No. 3, pp. 309-318, 2005) also can be used.

The fourth method is a method of generating a CS optical pulse train bymode-lock-operating a mode-locked DBR (Distributed Bragg Reflector)laser while adjusting the longitudinal mode wavelength of the resonatormode thereof, so that mode-locking operation arises in a longitudinalmode formed from only two wavelength components that have equalintensities (refer to S. Arahira, H. Yaegashi, K. Nakamura, and Y.Ogawa, “Generation of carrier-suppressed broad pulses from model lockedDBR laser operating with two carrier wavelengths”, Electronics Letters,12 Oct. 2006, vol. 42, No. 21, pp. 1298-1300). In accordance with thefourth method, it is possible to generate a CS optical pulse train byusing a single element, and the device can be made to be more compactand less expensive. Further, because the pulse duration of the opticalpulse structuring the CS optical pulse train can be changed in a widerange, the pulse duration of the optical pulse can be set flexibly inaccordance with the communications system that is used or the like.

However, the following problems to be solved exist in the CS opticalpulse train generating methods of the above-described first throughfourth related art.

In accordance with the first method, because a continuance wave lightsource is required separately from the LN optical intensity modulator,the device itself becomes large. Further, the amplitude Vpp of themodulation voltage required by the LN optical intensity modulator is 2Vπ, where Vπ is the half-wave voltage of the LN optical intensitymodulator. The half-wave voltage Vπ of a general LN optical intensitymodulator is 5 V to 10 V, and therefore, the amplitude Vpp of themodulation voltage is 10 V to 20 V. When converting to electric powerwith the impedance of the LN optical intensity modulator being 50Ω, thisis a large value of 24 dBm to 30 dBm. Accordingly, the first method is amethod necessitating a large amount of consumed electric power.

Supposing a case of utilization in a wavelength multiplex system or thelike, a large number of CS optical pulse train generating light sources,which corresponds to the wavelength multiplex number, is required.Accordingly, the amount of consumed electric power being large meansthat an amount of electric power that increases drastically inaccordance with the increase in the number of these CS optical pulsetrain generating light sources, is necessary. Due thereto, the systemitself must become large.

In accordance with the second method, there are the advantages that itis possible to generate a CS optical pulse train by using a singleelement, and the device can be made to be more compact and lessexpensive. However, in principle, only a sine wave optical pulse traincan be obtained, and flexible setting of the pulse width in accordancewith the system specifications cannot be carried out. Further, thecontrollable width of the wavelength is about several nm which isextremely narrow, and the usable range in practical use is limited.

In accordance with the third method, an optical pulse light source, thathas a repetition frequency of a magnitude that is half of the repetitionfrequency of the CS optical pulse train to be generated, is needed. Forexample, when generating a CS optical pulse train of a repetitionfrequency of 40 GHz, an optical pulse light source having a repetitionfrequency of 20 GHz is needed.

Further, with an optical delay interferometer that is needed in opticalphase control between optical pulse trains that have been branched intwo, there is the need for highly-precise adjustment corresponding tothe order of μm with respect to the optical path lengths of the twobranched optical pulse trains. Namely, the structure of the devicebecomes complex, and a highly-precise optical path length controllingcircuit is required. As a result, a device for realizing the thirdmethod is large and expensive.

In accordance with the fourth method, the wavelength width over whichthe wavelength of the CS optical pulse train to be generated can bevaried is about several nm which is narrow. Supposing a case in whichthe CS optical pulse train is applied to a large-capacity communicationssystem in accordance with a wavelength multiplexing method, it isdesirable that the wavelength variable range of the optical signalsource be such that the wavelength can be varied at least about one bandof the frequency band, due to requirements such as combining thewavelength into the prescribed wavelength grid of the system, ensuring aspare light source, and the like. For example, because the frequencyband width of the C band is 1535 nm to 1565 nm, realization of theability to vary the wavelength in a width of about the frequency bandwidth of this C band is desired of a CS optical pulse train generatingdevice.

SUMMARY OF THE INVENTION

The present invention provides a CS optical pulse train generatingdevice in which the optical pulse width, the duty ratio, and the centralwavelength can be varied. Further, the present invention provides a CSoptical pulse train generating device that can be made compact and inwhich the amount of consumed electric power is low. Moreover, thepresent invention provides a CS optical pulse train generating methodusing the CS optical pulse train generating device.

A first aspect of the present invention is a carrier-suppressed opticalpulse train generating device including: a first electric modulationsignal generator generating and outputting a first electric modulationsignal that is synchronous with a clock signal; a second electricmodulation signal generator generating and outputting a second electricmodulation signal of a same frequency as the first electric modulationsignal and to which a phase difference of δ radians is provided, where δis a real number satisfying 0≦δ≦π; a two-mode beat light source drivenby the first electric modulation signal, and generating and outputtingtwo-mode beat light; and an optical intensity modulator to which thetwo-mode beat light is inputted, and that optical-intensity-modulatesthe two-mode beat light, and generates and outputs a carrier-suppressedoptical pulse train having numerous longitudinal modes whoselongitudinal mode spectra are greater than 2, wherein lighttransmittance of the optical intensity modulator is modulated by thesecond electric modulation signal.

A second aspect of the present invention is a carrier-suppressed opticalpulse train generating device including: a first electric modulationsignal generator generating and outputting a first electric modulationsignal that is synchronous with a clock signal; a second electricmodulation signal generator generating and outputting a second electricmodulation signal of a same frequency as the first electric modulationsignal and to which a phase difference of δ radians is provided, where δis a real number satisfying 0≦δ≦π; and a Bragg reflection semiconductorlaser, wherein the Bragg reflection semiconductor laser comprises: firstand second sampled grating regions at which are formed sampled gratingsthat are structured such that a short-period grating is incorporated-inwithin one period of a long-period grating, and that have a doubleperiod structure of a long period and a short period; first and secondoptical intensity modulating regions having a function of modulatingoptical intensity; a gain region at which an inverted distribution isformed; and first and second phase adjusting regions at which anequivalent refractive index is variable, wherein a Bragg reflectionsemiconductor laser structure is formed by disposing, in series, thefirst optical intensity modulating region, the gain region and the firstand second phase adjusting regions, between the first sampled gratingregion and the second sampled grating region, the second opticalintensity modulating region is outside of a region sandwiched by thefirst sampled grating region and the second sampled grating region, andis structured by being disposed in series and adjacent to either one ofthe first sampled grating region and the second sampled grating region,a wavelength of oscillation light of a Bragg reflection semiconductorlaser structural portion can be varied by changing equivalent refractiveindices of the first and second sampled grating regions and the firstand second phase adjusting regions, the laser is mode-lock-operated bymodulating light transmittance of the first optical intensity modulatingregion by the first electric modulation signal, and can be made tooutput a carrier-suppressed optical pulse train, and a duty ratio of anoptical pulse structuring the carrier-suppressed optical pulse train canbe controlled by modulating light transmittance of the second opticalintensity modulating region by the second electric modulation signal.

A third aspect of the present invention is a carrier-suppressed opticalpulse train generating method including: a first electric modulationsignal generating step generating and outputting, by a first electricmodulation signal generator, a first electric modulation signal that issynchronous with a clock signal; a second electric modulation signalgenerating step generating and outputting, by a second electricmodulation signal generator, a second electric modulation signal of asame frequency as the first electric modulation signal and having aphase difference of δ radians, where δ is a real number satisfying0≦δ≦π; a two-mode beat light generating step driving a two-mode beatlight source by the first electric modulation signal, and generating andoutputting two-mode beat light that is synchronous with the clocksignal; and an optical intensity modulating stepoptical-intensity-modulating the two-mode beat light by an opticalintensity modulator that is driven by the second electric modulationsignal, and generating and outputting a carrier-suppressed optical pulsetrain having numerous longitudinal modes whose longitudinal mode spectraare greater than 2.

A fourth aspect of the present invention is a carrier-suppressed opticalpulse train generating method using a Bragg reflection semiconductorlaser including: first and second sampled grating regions at which areformed sampled gratings that are structured such that a short-periodgrating is incorporated-in within one period of a long-period grating,and that have a double period structure of a long period and a shortperiod; first and second optical intensity modulating regions having afunction of modulating optical intensity; a gain region at which aninverted distribution is formed; and first and second phase adjustingregions at which an equivalent refractive index is variable, where aBragg reflection semiconductor laser structure is formed by disposing,in series, the first optical intensity modulating region, the gainregion and the first and second phase adjusting regions, between thefirst sampled grating region and the second sampled grating region, thesecond optical intensity modulating region is outside of a regionsandwiched by the first sampled grating region and the second sampledgrating region, and is structured by being disposed in series andadjacent to either one of the first sampled grating region and thesecond sampled grating region, a wavelength of oscillation light of aBragg reflection semiconductor laser structural portion can be varied bychanging equivalent refractive indices of the first and second sampledgrating regions and the first and second phase adjusting regions, thelaser is mode-lock-operated by modulating light transmittance of thefirst optical intensity modulating region, and can be made to output acarrier-suppressed optical pulse train, and a duty ratio of an opticalpulse structuring the carrier-suppressed optical pulse train can becontrolled by modulating light transmittance of the second opticalintensity modulating region, the method including: a first electricmodulation signal generating step generating and outputting, by a firstelectric modulation signal generator, a first electric modulation signalthat is synchronous with a clock signal; a second electric modulationsignal generating step generating and outputting, by a second electricmodulation signal generator, a second electric modulation signal of asame frequency as the first electric modulation signal and having aphase difference of δ radians; a wavelength adjusting step varying awavelength of oscillation light at a Bragg reflection semiconductorlaser structural portion, by changing equivalent refractive indices ofthe first and second sampled grating regions and the first and secondphase adjusting regions; a mode-lock operating step causing mode-lockingoperation by modulating light transmittance of the first opticalintensity modulating region by the first electric modulation signal; anda duty ratio adjusting step controlling a duty ratio of an optical pulsestructuring a carrier-suppressed optical pulse train by modulating lighttransmittance of the second optical intensity modulating region by thesecond electric modulating signal.

BRIEF DESCRIPTION OF THE DRAWINGS

An exemplary embodiment of the present invention will be described indetail based on the following figures, wherein:

FIG. 1 is a schematic block structural diagram of a first CS opticalpulse train generating device of an exemplary embodiment of the presentinvention;

FIG. 2A through FIG. 2C are drawings showing time waveforms of two-modebeat light, a second electric modulation signal and a CS optical pulsetrain, respectively;

FIG. 2D and FIG. 2E are drawings showing frequency spectra of thetwo-mode beat light and the CS optical pulse train, respectively;

FIG. 3A is a drawing showing a time waveform of two-mode beat light in acase in which a phase difference between a first electric modulationsignal and the second electric modulation signal is 0 radians;

FIG. 3B is a drawing showing a time waveform of light transmittance ofan optical intensity modulator in a case in which the phase differencebetween the first electric modulation signal and the second electricmodulation signal is 0 radians;

FIG. 3C is a drawing showing a time waveform of a CS optical pulse trainin a case in which the phase difference between the first electricmodulation signal and the second electric modulation signal is 0radians;

FIG. 4A is a drawing showing a time waveform of two-mode beat light in acase in which the phase difference between the first electric modulationsignal and the second electric modulation signal is π radians;

FIG. 4B is a drawing showing a time waveform of light transmittance ofthe optical intensity modulator in a case in which the phase differencebetween the first electric modulation signal and the second electricmodulation signal is π radians;

FIG. 4C is a drawing showing a time waveform of a CS optical pulse trainin a case in which the phase difference between the first electricmodulation signal and-the second electric modulation signal is πradians;

FIG. 5A through FIG. 5C are drawings showing the time waveform of theenvelope of electric field of light of two-mode beat light, a timewaveform of light transmittance of the optical intensity modulator, anda time waveform of an envelope of electric field of light of a CSoptical pulse train, respectively;

FIG. 6A and FIG. 6B are drawings showing optical intensity timewaveforms of CS optical pulse trains, which are provided for explanationof the features of the optical intensity time waveforms and frequencyspectra of CS optical pulse trains on the basis of differences in theextinction ratios of the optical intensity modulator;

FIG. 6C and FIG. 6D are drawings showing the frequency spectra of the CSoptical pulse trains, which are provided for explanation of the featuresof the optical intensity time waveforms and the frequency spectra of theCS optical pulse trains on the basis of differences in the extinctionratios of the optical intensity modulator;

FIG. 7A is a drawing showing dependence, on the extinction ratio of theoptical intensity modulator, of the duty ratio of a CS optical pulsetrain, in a case in which a value of δ that provides a phase differencebetween the first electric modulation signal and the second electricmodulation signal is 0 radians;

FIG. 7B is a drawing showing dependence, on the extinction ratio of theoptical intensity modulator, of the duty ratio of a CS optical pulsetrain, in a case in which the value of δ that provides a phasedifference between the first electric modulation signal and the secondelectric modulation signal is π radians;

FIG. 8A is a drawing showing the relationship of light output intensitywith respect to applied voltage VEA of an semiconductorelectroabsorption intensity modulator, that is provided for explanationof an adjustable/variable range of the duty ratio in a case in which anon-linear optical intensity modulator is used;

FIG. 8B and FIG. 8C are drawings showing optical intensity timewaveforms of CS optical pulse trains that are generated and outputtedwhen an semiconductor electroabsorption intensity modulator is driven bya sine-wave-shaped second electric modulation signal, that are providedfor explanation of the adjustable/variable range of the duty ratio in acase in which the non-linear optical intensity modulator is used;

FIG. 9 is a schematic structural diagram of a second CS optical pulsetrain generating device of an exemplary embodiment of the presentinvention;

FIG. 10A is a drawing showing a sampled grating equivalent refractiveindex distribution along the lengthwise direction of an opticalwaveguide that is the propagating direction of laser light, that isprovided for explanation of the structure and function of sampledgratings that structure a first sampled grating region and a secondsampled grating region, respectively;

FIG. 10B is a drawing showing the reflecting characteristic inaccordance with a Bragg reflecting structure in a case in which theperiod structure of the refractive index is Λ, that is provided forexplanation of the structure and function of the sampled gratings thatstructure the first sampled grating region and the second sampledgrating region, respectively;

FIG. 11A through FIG. 11D are drawings showing a case in which the Braggreflection wavelengths of the Bragg reflectances of the first and secondsampled grating regions are the same, that are provided for explanationof the mechanism by which the wavelength of two-mode beat light, that isgenerated in an optical resonator of a Bragg reflection semiconductorlaser, is determined; and

FIG. 12A through FIG. 12D are drawings showing a case in which the Braggreflection wavelengths of the first and second sampled grating regionsare different, that are provided for explanation of the mechanism bywhich the wavelength of two-mode beat light, that is generated in anoptical resonator of a Bragg reflection semiconductor laser, isdetermined.

DETAILED DESCRIPTION OF THE INVENTION

The inventors of the invention studied methods of making the opticalpulse width of a CS optical pulse train variable, by focusing on thefact that two-mode beat light, that has a longitudinal mode spectrumstructured by only two longitudinal modes of equal intensities, has asine-wave-shaped time waveform. As a result, the inventors arrived atchanging the optical pulse width by optical-intensity-modulatingtwo-mode beat light by an optical intensity modulator under a particularcondition. Namely, the particular condition is the condition of setting,to a value of what extent, the phase difference between the phase of acontrol signal for optical-intensity-modulating the two-mode beat lightand the phase of the two-mode beat light. By methods such as computersimulation and the like, the inventors confirmed that the optical pulsewidth of a CS optical pulse train can be narrowed by making this phasedifference be 0, and can be widened by making this phase difference beπ.

In accordance with the gist of the invention of achieving the object ofthe optical pulse width, the duty ratio and the central wavelength beingvariable on the basis of the above-described guiding principle that theoptical pulse width of a CS optical pulse train is controlled byoptical-intensity-modulating two-mode beat light under a particularcondition, there is provided a CS optical pulse train generating deviceand a CS optical pulse train generating method of the followingstructures.

A first CS optical pulse train generating device of the invention isstructured to include a first electric modulation signal generator, asecond electric modulation signal generator, a two-mode beat lightsource, and an optical intensity modulator.

The first electric modulation signal generator generates and outputs afirst electric modulation signal that is synchronous with a clocksignal. The second electric modulation signal generator generates andoutputs a second electric modulation signal of a same frequency as thefirst electric modulation signal and to which a phase difference of δradians is provided. The two-mode beat light source is driven by thefirst electric modulation signal, and generates and outputs two-modebeat light that is synchronous with a clock signal. The two-mode beatlight is inputted to the optical intensity modulator, and, byoptical-intensity-modulating the two-mode beat light, the opticalintensity modulator generates and outputs a CS optical pulse trainhaving numerous longitudinal modes whose longitudinal mode spectra aregreater than 2. The light transmittance of the optical intensitymodulator is modulated by the second electric modulation signal.Further, δ is a real number that satisfies 0≦δ≦π.

Two-mode beat light means a CS optical pulse train whose longitudinalmode spectrum (wavelength spectrum) has only longitudinal modecomponents of two wavelengths whose intensities are equal to oneanother. On the other hand, even in cases in which the longitudinal modespectrum is formed from numerous longitudinal mode components exceeding2 wavelength components, it is a CS optical pulse train if it is anoptical pulse train in which the phases, that serve as the electricfield of light of optical pulses that are adjacent in the intensity timewaveform thereof, differ by π. Accordingly, a CS optical pulse train isa wide concept that includes two-mode beat light.

By optical-intensity-modulating the two-mode beat light by the opticalintensity modulator, as will be described later, the longitudinal modespectrum thereof becomes a CS optical pulse train that is formed fromnumerous longitudinal mode components exceeding two wavelengthcomponents. Accordingly, in the following explanation, for simplicity,the light that is obtained by optical-intensity-modulating two-mode beatlight will not be called “modulated two-mode beat light”, and will becalled a CS optical pulse train.

If the first CS optical pulse train generating device of the inventionis utilized, a first CS optical pulse train generating method of theinvention that will be described hereinafter is realized.

The first CS optical pulse train generating method is structured toinclude a first electric modulation signal generating step, a secondelectric modulation signal generating step, a two-mode beat lightgenerating step, and an optical intensity modulating step.

The first electric modulation signal generating step is a step thatgenerates and outputs, by a first electric modulation signal generator,a first electric modulation signal that is synchronous with a clocksignal. The second electric modulation signal generating step is a stepthat generates and outputs, by a second electric modulation signalgenerator, a second electric modulation signal of a same frequency asthe first electric modulation signal and having a phase difference of δradians. The two-mode beat light generating step is a step that, bydriving a two-mode beat light source by the first electric modulationsignal, generates and outputs two-mode beat light that is synchronouswith a clock signal. The optical intensity modulating step is a stepthat optical-intensity-modulates the two-mode beat light by an opticalintensity modulator that is driven by the second electric modulationsignal, and generates and outputs a CS optical pulse train havingnumerous longitudinal modes whose longitudinal mode spectra are greaterthan 2.

A second CS optical pulse train generating device of the invention hasthe feature of being structured by a two-mode beat light source and anoptical intensity modulator being monolithically integrated at a samesemiconductor substrate, in the above-described first CS optical pulsetrain generating device. A Bragg reflection semiconductor laserstructure is employed in order to realize the function of the two-modebeat light source. Namely, the Bragg reflection semiconductor laser thatis used in the second CS optical pulse train generating device isstructured by a usual Bragg reflection semiconductor laser structureportion that mode-lock-operates, and a portion having the function of anoptical intensity modulator, being integrated monolithically. An opticalintensity modulating region, that is for carrying out optical intensitymodulation of the two-mode beat light, is attached to the usual Braggreflection semiconductor laser structure portion.

The light source that is used in the second CS optical pulse traingenerating device is, as described above, a light source having aparticular structure that includes a usual Bragg reflectionsemiconductor laser structure portion that mode-lock-operates, and aportion having the function of an optical intensity modulator. Here,this light source is simply called a Bragg reflection semiconductorlaser. Namely, the second CS optical pulse train generating device ofthe invention is structured to include a first electric modulationsignal generator, a second electric modulation signal generator, and aBragg reflection semiconductor laser.

The first electric modulation signal generator generates and outputs afirst electric modulation signal that is synchronous with a clocksignal. The second electric modulation signal generator generates andoutputs a second electric modulation signal of a same frequency as thefirst electric modulation signal and to which a phase difference of δradians is provided.

The Bragg reflection semiconductor laser has first and second sampledgrating regions, first and second optical intensity modulating regions,a gain region at which an inverted distribution is formed, and first andsecond phase adjusting regions at which an equivalent refractive indexis variable. The basic Bragg reflection semiconductor laser structure isstructured by the first and second sampled grating regions and the gainregion. By adding the first optical intensity modulating region to thisbasic Bragg reflection semiconductor laser structure, a mode-lockedsemiconductor laser structure is formed. Providing this mode-lockedsemiconductor laser structure further with a second optical intensitymodulating region for carrying out optical intensity modulation oftwo-mode beat light is the feature of the Bragg reflection semiconductorlaser that the second CS optical pulse train generating device has.

Sampled gratings, that have a double period structure of a long periodand a short period that is structured by a short-period grating beingincorporated-in within one period of a long-period grating, are formedat the first and second sampled grating regions. The first and secondoptical intensity modulating regions have the function of modulatingoptical intensity. Further, the first optical intensity modulatingregion, the gain region and the first and second phase adjusting regionsare disposed in series between the first sampled grating region and thesecond sampled grating region. Moreover, the second optical intensitymodulating region is outside of the region sandwiched by the firstsampled grating region and the second sampled grating region, and isdisposed in series and adjacent to either one of the first sampledgrating region and the second sampled grating region.

By mode-lock-operating the Bragg reflection semiconductor laser bymodulating the light transmittance of the first optical intensitymodulating region by the first electric modulation signal, the Braggreflection semiconductor laser can be made to output a CS optical pulsetrain.

The wavelength of the oscillation light of the Bragg reflectionsemiconductor laser can be varied by changing the equivalent refractiveindices of the first and second sampled grating regions and the firstand second phase adjusting regions. The duty ratio of an optical pulsestructuring the CS optical pulse train can be controlled by modulatingthe light transmittance of the second optical intensity modulatingregion by the second electric modulation signal.

In the second CS optical pulse train generating device, it is suitablethat the first optical intensity modulating region is within an opticalresonator that is formed by the first sampled grating region and thesecond sampled grating region, and is disposed at a position that is acenter of the optical resonator where a time, until an optical pulsethat has passed through the first optical intensity modulating region isBragg-reflected at the first sampled grating region and returns to thefirst optical intensity modulating region, and a time, until an opticalpulse that has passed through the first optical intensity modulatingregion is Bragg-reflected at the second sampled grating region andreturns to the first optical intensity modulating region, are equal toN/Δf.

Here, N is an integer of greater than or equal to 1, and Δf is arepetition frequency of an optical pulse of the CS optical pulse trainthat is an optical pulse train.

By using the second CS optical pulse train generating device of theinvention, the following second CS optical pulse train generating methodof the invention is realized.

The second CS optical pulse train generating method is a CS opticalpulse train generating method using a Bragg reflection semiconductorlaser, and is structured to include a first electric modulation signalgenerating step, a second electric modulation signal generating step, awavelength adjusting step, a mode-lock operating step, and a duty ratioadjusting step. Here, the Bragg reflection semiconductor laser is theBragg reflection semiconductor laser that the second CS optical pulsetrain generating device has.

The first electric modulation signal generating step is a step thatgenerates and outputs, by a first electric modulation signal generator,a first electric modulation signal that is synchronous with a clocksignal.

The second electric modulation signal generating step is a step thatgenerates and outputs, by a second electric modulation signal generator,a second electric modulation signal of a same frequency as the firstelectric modulation signal and having a phase difference of δ radians.

The wavelength adjusting step is a step that varies a wavelength ofoscillation light at a Bragg reflection semiconductor laser structuralportion, by changing equivalent refractive indices of the first andsecond sampled grating regions and the first and second phase adjustingregions. The Bragg reflection semiconductor laser structural portionindicates the optical resonator that is formed by the above-describedfirst sampled grating region and second sampled grating region. In thefollowing explanation, this portion will be called the Bragg reflectionsemiconductor laser structural portion or will be called the opticalresonator in accordance with the technical contents that are beingdescribed, but the structural portion of the invention that the bothindicate is the same portion.

The mode-lock operating step is a step that causes mode-lockingoperation by modulating light transmittance of the first opticalintensity modulating region by the first electric modulation signal.

The duty ratio adjusting step is a step that controls a duty ratio of anoptical pulse structuring a CS optical pulse train by modulating lighttransmittance of the second optical intensity modulating region by thesecond electric modulating section.

In the first CS optical pulse train generating device and first CSoptical pulse train generating method, in order to make small the dutyratio of the CS optical pulse train to be generated, it is suitable thatthe value of δ that provides the phase difference between the firstelectric modulation signal and the second electric modulation signal isset to 0.

Further, in order to make the duty ratio of the CS optical pulse trainto be generated even smaller, it is suitable to set the value of δ to 0,and to set the bias value and the value of the intensity amplitude ofthe second electric modulation signal such that the minimum value of thelight transmittance of the optical intensity modulator is 0.

In the first CS optical pulse train generating device and first CSoptical pulse train generating method, in order to make large the dutyratio of the CS optical pulse train to be generated, it is suitable thatthe value of δ that provides the phase difference between the firstelectric modulation signal and the second electric modulation signal isset to π.

Further, in order to make the duty ratio of the CS optical pulse trainto be generated even larger, it is suitable to set the value of δ to π,and to set the bias value and the value of the intensity amplitude ofthe second electric modulation signal such that the extinction ratio,that is defined as the ratio of the maximum value and the minimum valueof the light transmittance of the optical intensity modulator, is amaximum value of immediately before occurrence of a splitting phenomenonthat divides a peak of a single optical pulse structuring the CS opticalpulse train into plural peaks.

In accordance with the first CS optical pulse train generating device, aCS optical pulse train is generated by two-mode beat light, which isoutputted from the two-mode beat light source, beingoptical-intensity-modulated by the optical intensity modulator. Thetwo-mode beat light has a longitudinal mode spectrum structured by onlylongitudinal mode wavelength components whose intensities are equal, andhas a sine-wave-shaped time waveform.

The first electric modulation signal is synchronous with the clocksignal, and the two-mode beat light is generated by being driven by thefirst electric modulation signal. Therefore, the two-mode beat lightalso is synchronous with the clock signal. Namely, the peak position,that is the maximum position on the time axis of the time waveform ofthe two-mode beat light, and the peak position, that is the maximumvalue on the time axis of the time waveform of the first electricmodulation signal, coincide. On the other hand, because the opticalintensity modulator is driven by the second electric modulation signal,the peak position, that is the maximum position on the time axis of thetime waveform of the light transmittance of the optical intensitymodulator, and the peak position, that is the maximum position on thetime axis of the second electric modulation signal time waveform,coincide.

Accordingly, given that the phase difference between the first electricmodulation signal and the second electric modulation signal is δradians, if the value of δ is 0, the peak position on the time axis ofthe time waveform of the two-mode beat light, and the peak position onthe time axis of the time waveform of the light transmittance of theoptical intensity modulator, coincide. Further, if the value of δ is π,the peak position on the time axis of the time waveform of the two-modebeat light, and the position of the minimum on the time axis of the timewaveform of the light transmittance of the optical intensity modulator,coincide.

Although the reason therefore will be described in detail later, theoptical pulse width of the CS optical pulse train is most narrow whenthe value of δ is 0, and is most wide when the value of δ is π. Namely,by varying the value of δ in the range from 0 to π, the optical pulsewidth of the CS optical pulse train can be controlled, and accordingly,the duty ratio can be changed.

The value of δ can be set arbitrarily by, for example, the known methodof controlling the first electric modulation signal generator and thesecond electric modulation signal generator by a common clock signal,and adjusting, by a delay device, the delay amount of the electric pulsesignal outputted from the second electric modulation signal generator.Further, the method of setting the value of δ is not limited to thismethod, and it is also possible to use the known method of adjusting thephase difference of the clock signals that are supplied to the firstelectric modulation signal generator and the second electric modulationsignal generator. In either case, if it is possible to vary the phasedifference between the first electric modulation signal and the secondelectric modulation signal in the range from 0 to π, any known techniquemay be used, and which technique to use falls within a matter of design.

In order to effectively control the pulse duration of the optical pulseof the CS optical pulse train to be generated, it is effective to adjustthe value of δ, and to control the characteristic of the lighttransmittance of the optical intensity modulator by adjusting the biasvalue and the value of the intensity amplitude of the second electricmodulation signal as described above. This has been made clear fromresearch based on computer simulation and the like.

On the other hand, when employing, as the two-mode beat light source, alight source of a form that phase-synchronously drives two semiconductorlasers that oscillate in a longitudinal single mode and combines andoutputs the two output lights that are outputted from these twosemiconductor lasers, the CS optical pulse train generating device atwhich the wavelength can be varied over a wide range is realized byvarying the oscillation wavelengths of the two semiconductor lasers.

Further, the CS optical pulse train generating device at which thewavelength can be varied over a wide range is realized also by using, asthe two-mode beat light source, a light source that is structured suchthat two-mode beat light is obtained by extracting, by a wavelengthfilter, only adjacent two wavelength components among the longitudinalmode components of outputted light of a mode-locked semiconductor laserhaving numerous longitudinal mode components. Namely, by changing thewavelengths of the two wavelength components that are extracted, thewavelength of the CS optical pulse train to be generated can be changed.Accordingly, the greater the number of wavelength components that areincluded in the longitudinal mode of a mode-locked semiconductor laser,the wider the variable wavelength range can be made to be.

The second CS optical pulse train generating device uses the Braggreflection semiconductor laser in which the function as a two-mode beatlight source and the function as an optical intensity modulator are madeintegral. Accordingly, in accordance with the second CS optical pulsetrain generating device, a CS optical pulse train generating device thatcan be made compact and at which a small amount of consumed electricpower suffices is realized.

An exemplary embodiment of the invention will be described hereinafterwith reference to the drawings. Note that the respective drawings thatare provided for explanation of the forms of the device illustratestructural examples relating to the invention, and merely schematicallyshow the arrangement relationships of the respective structural elementsto the extent that the invention can be understood, but the presentinvention is not limited to the illustrated examples.

<First CS Optical Pulse Train Generating Device> Structure

The structure of a first CS optical pulse train generating devicerelating to the exemplary embodiment of the invention will be describedwith reference to FIG. 1 and FIG. 2A through FIG. 2E.

FIG. 1 is a schematic block structural diagram of the first CS opticalpulse train generating device of the exemplary embodiment of theinvention. In FIG. 1, the propagation path of an optical pulse train isshown by the thick lines, and the propagation path of electric signalsis shown by the thin lines. The propagation path of the optical pulsetrain is a spatially joined path that is structured by combining opticalwaveguides such as optical fibers or the like, or optical elements suchas lenses or the like. How to concretely form the propagation path ofthe optical pulse train is a matter of design and is not essentialtechnical matter of the invention, and therefore, description thereof isomitted.

The first CS optical pulse train generating device of the presentexemplary embodiment is structured to include a first electricmodulation signal generator 10, a second electric modulation signalgenerator 14, a two-mode beat light source 18 and an optical intensitymodulator 20. Further, the first CS optical pulse train generatingdevice has a clock signal generator 22 that supplies clock signals tothe first electric modulation signal generator 10 and the secondelectric modulation signal generator 14.

The first electric modulation signal generator 10 executes a firstelectric modulation signal generating step of generating and outputtinga first electric modulation signal 13 that is synchronous with a clocksignal 23. The second electric modulation signal generator 14 executes asecond electric modulation signal generating step of generating andoutputting a second electric modulation signal 15 that is the samefrequency as the first electric modulation signal and to which a phasedifference of δ radians is provided. The time waveforms of the firstelectric modulation signal 13 and the second electric modulation signal15 are not limited to sine-wave waveforms, and may be pulse-shapedwaveforms.

The clock signal 23 is a clock signal that is the reference foroperating the system, originally in the first CS optical pulse traingenerating device, and also in optical communications systems and thelike that are structured by using the first CS optical pulse traingenerating device. Therefore, the clock signal 23 is upon occasioncalled a system clock signal.

The two-mode beat light source 18 is driven by the first electricmodulation signal 13, and executes a two-mode beat light generating stepof generating and outputting two-mode beat light 19 that is synchronouswith the clock signal 23. The optical intensity modulator 20 executes anoptical intensity modulating step of the two-mode beat light 19 beinginputted to the optical intensity modulator 20, and the opticalintensity modulator 20 optical-intensity-modulating the two-mode beatlight 19 and generating and outputting a CS optical pulse train 21having numerous longitudinal mode components whose longitudinal modespectra exceed 2. The light transmittance of the optical intensitymodulator 20 is modulated by the second electric modulation signal 15.

Here, “synchronous” in the two-mode beat light generating step meansthat the frequency and phase of the first electric modulation signal 13,and the repetition frequency and phase of the intensity time waveform ofthe two-mode beat light 19, coincide. The repetition frequency of theintensity time waveform of the two-mode beat light 19 is equal to thefrequency difference of two frequency components in the frequencyspectrum of the two-mode beat light 19. Namely, “synchronous” means thatthe frequency difference of two frequency components in the frequencyspectrum of the two-mode beat light 19 coincide with the frequency ofthe first electric modulation signal 13.

The time waveforms and frequency spectra (longitudinal mode spectra) ofthe two-mode beat light 19, the second electric modulation signal 15 andthe CS optical pulse train 21 will be described with reference to FIG.2A through FIG. 2E. FIG. 2A through FIG. 2C are drawings showing thetime waveforms of the two-mode beat light 19, the second electricmodulation signal 15 and the CS optical pulse train 21, respectively.Further, FIG. 2D and FIG. 2E are drawings showing the frequency spectraof the two-mode beat light 19 and the CS optical pulse train 21,respectively.

In FIG. 2A through FIG. 2C, the horizontal axis shows time on anarbitrary scale, and optical intensity is shown in the vertical axisdirection on an arbitrary scale. Further, in FIG. 2D and FIG. 2E, thehorizontal axis shows optical frequency on an arbitrary scale, andoptical intensity is shown in the vertical axis direction on anarbitrary scale.

FIG. 2A is a drawing showing the time waveform of the optical intensityof the envelope of the electric field of light forming the two-mode beatlight 19. The time waveform, which is observed as changes in the opticalintensity, is expressed as the envelope of a time waveform obtained bysquaring the absolute value of the amplitude of the electric fieldvector of the light serving as the electric field of light. Accordingly,in the following explanation, “time waveform of the optical intensity ofthe optical pulse” or merely “time waveform of the optical pulse” meansthe envelope of the time waveform obtained by squaring the absolutevalue of the amplitude of the electric field vector of the light.

The two-mode beat light 19 is an optical pulse train in which the phasesserving as the electric field of light of the optical pulses that arelined-up adjacent on the time axis have the relationship of being phasesthat are opposite one another. Namely, there is the relationship thatthe phase difference between the both serving as electric field of lightof optical pulses that are lined-up adjacent on the time axis is equalto π. This is expressed by “0” and “π” alternately at the peak positionsof the optical pulses in FIG. 2A. Namely, this means that the phasesserving as the electric field of light of mutually adjacent opticalpulses differ by π.

As shown in FIG. 2D, the frequencies of the two frequency components ofthe frequency spectrum of the two-mode beat light 19 are f0+Δf/2 andf0−Δf/2. Here, f0 is the frequency serving as the electric field oflight of the two-mode beat light 19, and Δf is the repetition frequencyof the optical pulses of the two-mode beat light 19 that is an opticalpulse train. Further, the frequency serving as the electric field oflight of the CS optical pulse train 21 that is generated by the two-modebeat light 19, that is outputted from the optical intensity modulator20, being optical-intensity-modulated, is f0. The repetition frequencyof the optical pulses of the CS optical pulse train 21 is Δf.

As shown in FIG. 2B, the bias voltage of the second electric modulationsignal 15 is set to VDC, and the intensity amplitude is set to Vpp.Because the phases of the two-mode beat light 19 and the second electricmodulation signal 15 are illustrated as the same phases in FIG. 2A andFIG. 2B, the peak positions of the both time waveforms coincide.Generally, the phases of the two-mode beat light 19 and the secondelectric modulation signal 15 can be varied within a range from 0 to πradians.

As shown in FIG. 2C, the period and phase of the time waveform of the CSoptical pulse train 21 are the same as the two-mode beat light 19, butthe pulse duration of the optical pulses differs. FIG. 2C illustrates acase in which the pulse duration of the optical pulses is wide. However,the pulse duration of the optical pulses can be changed in accordancewith the extent to which the phase difference between the two-mode beatlight 19 and the second electric modulation signal 15 is set.

The difference that is clear from comparing the frequency spectra shownin FIG. 2D and FIG. 2E is as follows. Namely, the difference in therespective frequency spectrum structures of the two-mode beat light 19shown in FIG. 2D and the CS optical pulse train 21 shown in FIG. 2E isthat the frequency components of the frequency spectrum of the two-modebeat light 19 are the two components of f0+Δf/2 and f0−Δf/2, whereas thefrequency components of the CS optical pulse train 21 have, in additionto f0+Δf/2 and f0−Δf/2, numerous modulation sideband components offrequencies that are separated, by integer multiples of Δf, from thefrequencies of these frequency components.

Any of various light sources can be used as the two-mode beat lightsource 18 in accordance with the convenience thereof in terms of design.For example, the two-mode oscillation laser disclosed in theaforementioned publication can be used. Further, the combined output oftwo single longitudinal mode semiconductor lasers whose phases aresynchronized can also be used as the two-mode beat light source. In thiscase, distributed feedback semiconductor lasers (DFB lasers: distributedfeedback lasers) or Bragg reflection semiconductor lasers (DBR lasers:distributed Bragg reflector lasers) are used as the single longitudinalmode semiconductor lasers.

Further, cases in which two adjacent longitudinal mode components areextracted by a wavelength filter or the like from an optical pulse trainoutputted from a mode-locked laser that is mode-lock-operated by thefirst electric modulation signal 13, or from numerous longitudinal modecomponents having optical signals that are optical-intensity-modulatedor phase-modulated synchronously with the first electric modulationsignal 13 similarly, also are two-mode beat light sources.

The method for synchronizing the phases of the first electric modulationsignal 13 and the second electric modulation signal 15 and, further,making the phase difference between the both be δ radians, isimplemented as follows.

In a first method, first, the clock signal 23 that is outputted from theclock signal generator 22 is inputted to the first electric modulationsignal generator 10, and first electric modulation signals 11 and 13 ofthe same phase as the clock signal 23 are generated and outputted. Next,the first electric modulation signal 11 is inputted to the secondelectric modulation signal generator 14, a phase difference of δ radiansis added to the first electric modulation signal 11, and the secondelectric modulation signal 15 is generated and outputted. In this case,the second electric modulation signal 15 is generated on the basis ofthe first electric modulation signal 11 of the same phase as the firstelectric modulation signal 13. Therefore, the first electric modulationsignal 13 and the second electric modulation signal 15 are synchronous,and the phase difference between the both is δ radians.

In the first method, the second electric modulation signal generator 14has the function of adding a phase delay of δ radians to the firstelectric modulation signal 11 and generating the second electricmodulation signal 15. If this function of the second electric modulationsignal generator 14 is focused upon, the second electric modulationsignal generator 14 can be structured by a phase delay device havingonly a phase delaying function. However, as will be described later, inaddition to adding a phase delay to the inputted first electricmodulation signal, the second electric modulation signal generator 14must also have the function of changing the bias value and the value ofthe intensity amplitude of the first electric modulation signal.Accordingly, the second electric modulation signal generator 14 is not amere phase delay device, and must function to change the bias value andthe value of the intensity amplitude of the inputted electric signal.

Further, the first electric modulation signal 13 and the second electricmodulation signal 15 can be generated as follows. Namely, in a secondmethod, first, the clock signal 23 that is outputted from the clocksignal generator 22 is branched in two into a clock signal 23-1 and aclock signal 23-2 by a branching device 16. Next, the clock signal 23-1is inputted to the first electric modulation signal generator 10, andthe clock signal 23-2 is inputted to a delay device 24. At the delaydevice 24, a phase difference of δ radians is added to the clock signal23-2 and a clock signal 25 is generated, and this clock signal 25 isinputted to the second electric modulation signal generator 14. By doingso, the first electric modulation signal 13 of the same phase as theclock signal 23-1 is generated and outputted from the first electricmodulation signal generator 10. Further, the second electric modulationsignal 15 of the same phase as the clock signal 25 is generated andoutputted from the second-electric modulation signal generator 14. Inthe case of the second method, the clock signal 23-1 and the clocksignal 25 are made synchronous, and the phase difference between theboth is δ radians. Accordingly, the first electric modulation signal 13and the second electric modulation signal 15 are made synchronous, andthe phase difference between the both is δ radians.

A concrete example for generating the first electric modulation signal13 and the second electric modulation signal 15 will be described. Here,a case of generating a CS optical pulse train of a repetition frequencyof 40 GHz will be considered. However, it is obvious that cases ofgenerating CS optical pulse trains of repetition frequencies other than40 GHz can be implemented. When generating a CS optical pulse train of arepetition frequency of other than 40 GHz, instead of a 40 GHz electricsignal oscillator, it suffices to utilize an electric signal oscillatorthat generates and outputs an electric pulse signal that is equal to therepetition frequency of the CS optical pulse train to be generated.

First, an electric pulse signal of 40 GHz that is synchronous with thesystem clock signal (the clock signal 23) is generated by using a 40 GHZelectric signal oscillator. This 40 GHz electric signal oscillatorcorresponds to the first electric modulation signal generator 10. Next,the output from the 40 GHz electric signal oscillator is branched in twoat a power divider. One of the signals branched in two is inputted to aphase shifter.

When utilizing such a structure, the first electric modulation signal 13is outputted from the branch output port of the power divider that isnot connected to the phase shifter. Further, the second electricmodulation signal 15 is outputted from the output port of the phaseshifter. Namely, the first electric modulation signal 13 and the secondelectric modulation signal 15 can be generated and outputted by theabove-described first method.

In accordance with any of the above-described methods, the phases of thefirst electric modulation signal 13 and the second electric modulationsignal 15 can be made to be synchronous, and further, the phasedifference between the both can be made to be δ radians. In FIG. 1, theelectrical path used in the above-described first method is shown by theone-dot chain line, and the electrical path used in the second method isshown by the two-dot chain line.

Operation

Operation of the first CS optical pulse train generating device of thepresent exemplary embodiment will be described with reference to FIG. 3Athrough FIG. 3C and FIG. 4A through FIG. 4C. In FIG. 3A through FIG. 3Cand FIG. 4A through FIG. 4C, time is shown on the horizontal axis on anarbitrary scale, and optical intensity is shown on the vertical axis onan arbitrary scale.

FIG. 3A through FIG. 3C illustrate the light transmittances of thetwo-mode beat light 19 and the optical intensity modulator 20 and thetime waveform of the CS optical pulse train 21, respectively, in a casein which the phase difference between the first electric modulationsignal 13 and the second electric modulation signal 15 is 0 radians.FIG. 4A through FIG. 4C illustrate the light transmittances of thetwo-mode beat light 19 and the optical intensity modulator 20 and thetime waveform of the CS optical pulse train 21, respectively, in a casein which the phase difference between the first electric modulationsignal 13 and the second electric modulation signal 15 is 0 π radians.

Here, in order to explain the present exemplary embodiment simply, it issupposed that the time change of the light transmittance of the opticalintensity modulator 20 is provided by a sine wave. However, the presentexemplary embodiment is not limited to a case in which the time changeof the light transmittance of the optical intensity modulator 20 isprovided by a sine wave, and similar effects are obtained even in a casein which the time change is provided by a pulse-shaped waveform.

As shown in FIG. 3A and FIG. 3B, in a case in which the phase differenceof the first electric modulation signal 13 and the second electricmodulation signal 15 is 0 radians, the maximum position on the time axisof the time waveform of the two-mode beat light 19, and the maximumposition on the time axis of the time waveform of the lighttransmittance of the optical intensity modulator 20, coincide. The timewaveform of the CS optical pulse train 21 is provided by the product ofthe time waveform of the two-mode beat light 19 and the time waveform ofthe light transmittance of the optical intensity modulator 20.Accordingly, in a case in which the phase difference of the firstelectric modulation signal 13 and the second electric modulation signal15 is 0 radians, the CS optical pulse train 21 receives greater dampingof optical intensity at the optical intensity modulator 20 in a vicinityof the minimum as compared with a vicinity of the maximum of the timewaveform of the CS optical pulse train 21. Namely, in this case, at thetime waveform of the CS optical pulse train 21, a contracting phenomenonof the optical pulse width occurs.

The proportion of contraction of the optical pulse width varies inaccordance with the extinction ratio of the time waveform of the lighttransmittance of the optical intensity modulator 20, i.e., the ratio ofthe maximum value and the minimum value of the light transmittance ofthe optical intensity modulator 20 (the value obtained by dividing themaximum value by the minimum value). In most optical intensitymodulators that are used in practice, the extinction ratio of theoptical intensity modulation can be controlled by the intensitymodulation current or the intensity modulation voltage of the electricmodulation signal provided to the optical intensity modulator.Accordingly, by varying the bias value that is a DC component and thevalue of the intensity amplitude that is an AC component of the secondelectric modulation signal 15, the extinction ratio of the opticalintensity modulation can be controlled.

The second electric modulation signal 15 is a modulation signal providedby the sum of the DC component and the AC signal component. Therefore,the device can be structured by combining an electric pulse signalgenerator that generates an AC signal component, and a DC power sourcethat can supply constant current or constant voltage that generates a DCcomponent. Namely, it suffices to structure the device by combining andoutputting, at a combining device, an AC signal outputted from anelectric pulse signal generator and a DC component outputted from a DCpower source. Changing the bias value that is the DC component of thesecond electric modulation signal 15 can be handled by changing theoutput value of the DC power source. Further, changing the value of theintensity amplitude of the AC component can be handled by changing theintensity amplitude of the output value of the electric pulse signalgenerator.

On the other hand, in a case in which the phase difference of the firstelectric modulation signal 13 and the second electric modulation signal15 is π radians, as shown in FIG. 4A and FIG. 4B, the maximum positionon the time axis of the time waveform of the two-mode beat light 19, andthe minimum position on the time axis of the time waveform of the lighttransmittance of the optical intensity modulator 20, coincide. Also inthe case in which the phase difference of the first electric modulationsignal 13 and the second electric modulation signal 15 is π radians, asdescribed above, the time waveform of the CS optical pulse train 21 isprovided by the product of the time waveform of the two-mode beat light19 and the time waveform of the light transmittance of the opticalintensity modulator 20. Accordingly, in this case, the CS optical pulsetrain 21 receives greater damping of optical intensity at the opticalintensity modulator 20 in a vicinity of the maximum as compared with avicinity of the minimum of the time waveform of the CS optical pulsetrain 21. Namely, in this case, at the time waveform of the CS opticalpulse train 21, an expanding phenomenon of the optical pulse widthoccurs.

In the same way as in the above-described case of contraction of theoptical pulse width, the proportion of expansion of the optical pulsewidth as well varies in accordance with the extinction ratio of thelight transmittance of the optical intensity modulator 20. Accordingly,by varying the bias value that is a DC component and the value of theintensity amplitude that is an AC component of the second electricmodulation signal 15, the extinction ratio of the optical intensitymodulator can be controlled.

Namely, in either case of contraction of the optical pulse width orexpansion of the optical pulse width, the value of δ is set to 0, andthe extinction ratio of the optical intensity modulator can becontrolled by varying the relationship of the magnitudes of theproportion (t1/(t1+t2)) occupied by width t1 of a time band that takes avalue larger than an intermediate value between the maximum value andthe minimum value of the light transmittance, and the proportion(t2/(t1+t2)) occupied by width t2 of a time band that takes a valuesmaller than the intermediate value between the maximum value and theminimum value of the light transmittance, during one period(1/Δf=(t1+t2)) of the light transmittance of the optical intensitymodulator.

The switching of the contracting of the optical pulse width and theexpanding of the optical pulse width may be carried out by selectingwhether the maximum position on the time axis of the time waveform ofthe two-mode beat light 19 is made to match the maximum position or theminimum position on the time axis of the time waveform of the lighttransmittance of the optical intensity modulator 20. This selection iseasily executed by providing a phase difference of 0 or π, with respectto the first electric modulation signal 13, to the second electricmodulation signal 15.

The shape of the time waveform of the CS optical pulse train 21 will bedescribed with reference to FIG. 5A through FIG. 5C. FIG. 5A is adrawing showing the time waveform of the envelope of the electric fieldof light of the two-mode beat light 19. FIG. 5B is a drawing showing thetime waveform of the light transmittance of the optical intensitymodulator 20. FIG. 5C is a drawing showing the time waveform of theenvelope of the electric field of light of the CS optical pulse train21. Hereinafter, the time waveform of the envelope of the electric fieldof light will be called the amplitude waveform upon occasion.

In FIG. 5A through FIG. 5C, time is shown on the horizontal axis on anarbitrary scale. Further, the magnitude of the electric field vector ofthe light is shown on an arbitrary scale on the vertical axes of FIG. 5Aand FIG. 5C. The light transmittance of the optical intensity modulator20 is shown on the vertical axis of FIG. 5B on an arbitrary scale.

Because the two-mode beat light 19 is the most typical CS optical pulsetrain, the phase of the electric field of light at the maximum positionof the envelope of the electric field of light thereof, and the phase ofthe electric field of light at the minimum position, differ by π.Namely, the values of the phase of the electric field of light at themaximum positions and minimum positions are respectively 0, π, 0, π, . .. . The values of the amplitude of the electric field of light at themaximum positions and the minimum positions are in the relationship ofbeing positive and negative or negative and positive, respectively. Theamplitude waveform-of the envelope of the electric field of light of theCS optical pulse train 21 outputted from the optical intensity modulator20 is provided to the two-mode beat light 19 by the product of theamplitude waveform of the envelope of the electric field of light andthe square root of the light transmittance of the optical intensitymodulator 20.

Accordingly, in the same way as the two-mode beat light 19, also at theamplitude waveform of the envelope of the electric field of light of theCS optical pulse train 21, the values of the electric field vectors ofthe electric field of light at the maximum position and the minimumposition are in the relationship of being positive and negative ornegative and positive, respectively. Namely, even if the two-mode beatlight 19 is optical-intensity-modulated, the nature thereof as a CSoptical pulse train is not deteriorated.

The effects achieved by the first CS optical pulse train generatingdevice of the invention will be described on the basis of FIG. 6Athrough FIG. 6D, with reference to results of computer simulation. Inthe following description, for simplicity and to the extent that it doesnot cause misunderstanding, there are cases in which the first electricmodulation-signal generator 10, the second electric modulation signalgenerator 14, the optical intensity modulator 20 are not written assuch, and the numerals indicating the respective blocks such as 10, 14,20 that show the correspondence with the block structural diagram ofFIG. 1 are omitted, and these elements are written as the first electricmodulation signal generator, the second electric modulation signalgenerator, the optical intensity modulator.

FIG. 6A through FIG. 6D are drawings provided for explanation of theoptical intensity time waveform and the frequency spectrum of the CSoptical pulse train. FIG. 6A and FIG. 6B are drawings showing opticalintensity time waveforms of the CS optical pulse train, and time that isstandardized with respect to the period (1/Δf) of the clock signal ismarked on the horizontal axis. Further, optical intensity isstandardized with respect to the optical peak intensity and marked onthe vertical axis. FIG. 6C and FIG. 6D are drawings showing frequencyspectra of the CS optical pulse train. The frequency shift amount isstandardized and marked on the horizontal axis, and the opticalintensity is standardized and marked on the vertical axis. The frequencyshift amount is a value, in the longitudinal mode frequency spectrum,that shows the frequency f0 of the electric field of light of the CSoptical pulse train being coordinate-converted to 0. Namely, a valuecorresponding to Δf/2 for the frequency f0+Δf/2, and a valuecorresponding to −Δf/2 for the frequency f0−Δf/2 are written on thehorizontal axis.

FIG. 6A and FIG. 6C are drawings showing the optical intensity timewaveform and the frequency spectrum of the CS optical pulse trainrespectively, in a case in which the extinction ratio of the opticalintensity modulator is 10 dB and the phase difference δ between thefirst electric modulation signal and the second electric modulationsignal is set to 0 radians. Further, FIG. 6B and FIG. 6D are drawingsshowing the optical intensity time waveform and the frequency spectrumof the CS optical pulse train, in a case in which the extinction ratioof the optical intensity modulator is 3 dB and the phase difference δbetween the first electric modulation signal and the second electricmodulation signal is set to π radians. The extinction ratio of theoptical intensity modulator is the ratio of the maximum value and theminimum value of the light transmittance of the optical intensitymodulator. In FIG. 6A and FIG. 6B, the dashed lines show the opticalintensity time waveforms of the two-mode beat light, and the solid linesshow the optical intensity time waveforms of the CS optical pulse train.

As shown in FIG. 6A and FIG. 6C, in a case in which the phase differenceδ between the first electric modulation signal and the second electricmodulation signal is 0 radians, a CS optical pulse train in which thepulse duration of the optical pulse is contracted is generated. As aresult, the amplitude time waveform of the two-mode beat light is a sinewave. Therefore, the duty ratio of the optical intensity time waveformof the two-mode beat light is 50%, and the duty ratio of the opticalintensity time waveform of the generated CS optical pulse train is 37%.

On the other hand, as shown in FIG. 6B and FIG. 6D, in a case in whichthe phase difference δ between the first electric modulation signal andthe second electric modulation signal is π radians, a CS optical pulsetrain in which the pulse duration of the optical pulse is expanded isgenerated. As a result, the duty ratio of the optical intensity timewaveform of the generated CS optical pulse train is 64%, which is alarger value than the 50% duty ratio of the optical intensity timewaveform of the two-mode beat light.

Further, as shown in FIG. 6C and FIG. 6D, the frequency spectrum of thegenerated CS optical pulse train does not have a frequency componentserving as an electric field of light at the position where thefrequency shift amount is 0, and exhibits a waveform in which modulationsidebands of the interval Δf widen symmetrically to the left and theright. This shows that the optical pulse train outputted from theoptical intensity modulator is a CS optical pulse train.

As described above, in order to make the duty ratio of the CS opticalpulse train to be generated small, it suffices to set the value of δ,which provides the phase difference between the first electricmodulation signal and the second electric modulation signal, to 0.Further, in order to make the duty ratio large, it suffices to set thevalue of δ to π.

The effects provided to the operation of modulating the duty ratio ofthe CS optical pulse train generated by the first CS optical pulse traingenerating device, that are obtained by controlling the extinction ratioof the light transmittance of the optical intensity modulator, will beexplained with reference to FIG. 7A and FIG. 7B on the basis of resultsverified by computer simulation.

FIG. 7A and FIG. 7B are drawings showing the dependence of the dutyratio of the CS optical pulse train on the extinction ratio of theoptical intensity modulator. The extinction ratio is marked in dB on thehorizontal axis, and the duty ratio is marked on the vertical axis. FIG.7A shows the dependence, on the extinction ratio of the opticalintensity modulator, of the duty ratio of the CS optical pulse train ina case in which the value of δ that provides the phase differencebetween the first electric modulation signal and the second electricmodulation signal is 0 radians. Further, FIG. 7B shows the dependence,on the extinction ratio of the optical intensity modulator, of the dutyratio of the CS optical pulse train in a case in which the value of δ isπ radians.

As shown in FIG. 7A, when the value of δ is 0 radians, the duty ratiodecreases monotonically in accordance with the increase in theextinction ratio of the optical intensity modulator. When the limitvalue is determined in a case in which the extinction ratio is infinite,it is confirmed that the duty ratio can be reduced to 36%. When theextinction ratio is infinite, the minimum value of the lighttransmittance of the optical intensity modulator is 0.

Namely, in order to make the duty ratio of the CS optical pulse train tobe generated even smaller, it suffices to set the value of δ to 0, andto set the bias value and the value of the intensity amplitude of thesecond electric modulation signal such that the minimum value of thelight transmittance of the optical intensity modulator becomes 0.

On the other hand, as shown in FIG. 7B, when the value of δ is πradians, the duty ratio increases monotonically in accordance with theincrease in the extinction ratio of the optical intensity modulator. Inthis case, the greater the extinction ratio of the optical intensitymodulator is set, the greater the duty ratio can be made to be. However,in this case, there is an upper limit on the magnitude of the duty ratiothat can be set.

Namely, when the extinction ratio of the optical intensity modulator isincreased, a splitting phenomenon arises in which the peak of a singleoptical pulse structuring the generated CS optical pulse train isdivided into plural peaks. The single optical pulse structuring the CSoptical pulse train whose peak is divided into plural peaks cannot beused as a CS optical pulse train for generating an optical pulse signalof the usual CS-RZ format.

In a case in which the time change that provides changes in the lighttransmittance of the optical intensity modulator is a sine wave, thevalue of the maximum extinction ratio that is immediately before theoccurrence of the splitting phenomenon of the single optical pulsestructuring the CS optical pulse train that is generated and outputtedat the optical intensity modulator is 3.5 dB. The duty ratio of the CSoptical pulse train obtained by setting the value of the extinctionratio to 3.5 dB is 66%.

Namely, in order to make the duty ratio of the CS optical pulse train tobe generated even larger, it suffices to set the value of δ to π, and toset the bias value and the value of the intensity amplitude of thesecond electric modulation signal such that the extinction ratio of theoptical intensity modulator becomes the maximum value of immediatelybefore the occurrence of the splitting phenomenon that divides the peakof a single optical pulse structuring the CS optical pulse train intoplural peaks.

In accordance with the first CS optical pulse train generating device ofthe present exemplary embodiment, by adjusting the value of δ andadjusting the extinction ratio of the optical intensity modulator, it ispossible to adjust/change, in the range of 36% to 66%, the duty ratio ofthe CS optical pulse train that is generated under the condition thatthe time change providing the change in the light transmittance of theoptical intensity modulator is a sine wave.

The range in which the duty ratio can be adjusted/changed can bebroadened more by providing, by a non-sinusoidal wave, the time changethat provides the change in the light transmittance of the opticalintensity modulator. This is realized by using an optical intensitymodulator that is a voltage-control-type optical intensity modulator andthat has the characteristic that the voltage dependence of the lighttransmittance is provided by a non-linear relationship. An example of anoptical intensity modulator having such an optical intensity modulatingcharacteristic is a semiconductor electroabsorption intensity modulator.

The range over which the duty ratio can be adjusted/changed, that isobtained in a case using an optical intensity modulator having thecharacteristic that the voltage dependence of the light transmittance isprovided by a non-linear relationship (hereinafter called a non-linearoptical intensity modulator upon occasion), will be described withreference to FIG. 8A through FIG. 8C. FIG. 8A through FIG. 8C aredrawings provided for explaining the duty ratio adjustable/changeablerange in a case in which a non-linear optical intensity modulator isused.

FIG. 8A is a drawing showing the relationship of light output intensitywith respect to applied voltage VEA of a semiconductor electroabsorptionintensity modulator. The applied voltage VEA is marked on the horizontalaxis in units of volts, and the output light intensity is marked in dBon the vertical axis. FIGS. 8B and 8C are drawings showing the opticalintensity time waveform of the CS optical pulse train that is generatedand outputted in a case in which the semiconductor electroabsorptionintensity modulator is driven by the second electric modulation signalthat is sine-wave-shaped.

In FIG. 8B, the bias voltage of the second electric modulation signal is−0.5 V, a value Vpp of the intensity amplitude is 1 V, and the value ofthe phase difference δ between the first electric modulation signal andthe second electric modulation signal is set to π radians. Further, inFIG. 8C, the bias voltage of the second electric modulation signal is−3.0 V, the value Vpp of the intensity amplitude is 3 V, and the valueof δ is set to 0 radians.

By setting the bias voltage and the value Vpp of the intensity amplitudeof the second electric modulation signal to the aforementioned valuesrespectively, the semiconductor electroabsorption intensity modulator ismade to operate as a non-linear optical intensity modulator.

In FIG. 8B and FIG. 8C, the dashed line shows the optical intensity timewaveform of the two-mode beat light, and the solid line shows theoptical intensity time waveform of the generated CS optical pulse train.

A case in which a CS optical pulse train, whose duty ratio is themaximum, is generated is shown in FIG. 8B, and the duty ratio is 69%.Further, a case in which a CS optical pulse train, whose duty ratio isthe minimum, is generated is shown in FIG. 8C, and the duty ratio is26%.

Namely, in accordance with the first CS optical pulse train generatingdevice of the present exemplary embodiment, by adjusting the value of δand by adjusting the extinction ratio of the optical intensitymodulator, it is possible to adjust/change the duty ratio of the CSoptical pulse train to be generated in the range of 26% to 69%, underthe condition that the time change that provides the change in the lighttransmittance of the optical intensity modulator is a non-sinusoidalwave. Due thereto, the range of adjusting/changing the duty ratio can bebroadened even more by changing the time change, that provides thechange in the light transmittance of the optical intensity modulator,from a sine wave to a non-sinusoidal wave.

<Second CS Optical Pulse Train Generating Device> Structure

The structure of a second CS optical pulse train generating device ofthe present exemplary embodiment will be described with reference toFIG. 9. FIG. 9 is a schematic structural diagram of the second CSoptical pulse train generating device of the exemplary embodiment of theinvention.

The second CS optical pulse train generating device of the presentexemplary embodiment is structured to include a first electricmodulation signal generator 70, a second electric modulation signalgenerator 90, and a Bragg reflection semiconductor laser 30.

The first electric modulation signal generator 70 generates and outputsa first electric modulation signal 73 that is synchronous with a clocksignal. The second electric modulation signal generator 90 generates andoutputs a second electric modulation signal 93 that is the samefrequency as the first electric modulation signal 73 and to which aphase difference of δ radians is provided.

The method of synchronizing the phases of the first electric modulationsignal 73 and the second electric modulation signal 93 and making thephase difference between the both be δ radians is similar to the case inthe first CS optical pulse train generating device, and therefore,repeat description will be omitted. In FIG. 9, it is assumed that thefirst electric modulation signal 73 and the second electric modulationsignal 93 are generated by the above-described first method, andsimplified illustration is given. Note that the device may be structuredto generate the first electric modulation signal 73 and the secondelectric modulation signal 93 in accordance with the above-describedsecond method.

The Bragg reflection semiconductor laser 30 has a first sampled gratingregion 40 and a second sampled grating region 50, a first opticalintensity modulating region 44 and a second optical intensity modulatingregion 52, a gain region 46 at which an inverted distribution is formed,and a first phase adjusting region 42 and a second phase adjustingregion 48 at which the equivalent refractive index can be varied.Hereinafter, when referring to both the first sampled grating region 40and the second sampled grating region 50, they will be simply called the“sampled grating regions” upon occasion, to the extent that nomisunderstanding arises.

At the first optical intensity modulating region 44 and the secondoptical intensity modulating region 52, an electric field absorbingeffect is manifested due to inverse bias voltage being applied, andoptical intensity modulation is thereby realized.

The basic structure of the Bragg reflection semiconductor laser 30 is astructure in which an optical waveguide layer 36 is sandwiched between ap-side cladding layer 38 and an n-side cladding layer 34, and is acurrent-injection-type semiconductor laser in accordance with the p-njunction. The aforementioned sampled grating regions, first opticalintensity modulating region 44 and second optical intensity modulatingregion 52, gain region 46 at which an inverted distribution is formed,and first phase adjusting region 42 and second phase adjusting region 48at which the equivalent refractive index can be varied, are respectivelystructured by an optical waveguide and the double hetero structure ofthe p-side cladding layer 38 and the n-side cladding layer 34 thatsandwich the optical waveguide.

The demarcating of the aforementioned respective regions is carried outby electrodes that are structured so as to contact the p-side claddinglayer. Different power sources corresponding to the respective functionsare connected to the above respective regions, and the respectivefunctions are manifested by electric signals supplied from these powersources. Accordingly, the above respective regions are structured toinclude the p-side cladding layer 38 and the n-side cladding layer 34that sandwich the optical waveguide layer 36, and respective p-sideelectrodes and an n-side common electrode 32. However, for convenienceof explanation, the respective regions are designated by indicating theoptical waveguide existing in the corresponding region.

Namely, at the first sampled grating region 40, the equivalentrefractive index is adjusted by a control electric signal being suppliedfrom a power source 74 via a p-side electrode 54 of the first sampledgrating region. At the second sampled grating region 50, the equivalentrefractive index is adjusted by a control electric signal being suppliedfrom a power source 84 via a p-side electrode 64 of the second sampledgrating region. The first optical intensity modulating region 44 isdriven by the first electric modulation signal 73 being supplied fromthe first electric modulation signal generator 70 via a p-side electrode58 of the first optical intensity modulating region. The second opticalintensity modulating region 52 is driven by the second electricmodulation signal 93 being supplied from the second electric modulationsignal generator 90 via a p-side electrode 66 of the second opticalintensity modulating region. At the gain region 46, a populationinversion is formed due to injection current being supplied from a powersource 80 via a p-side electrode 60 of the gain region. A modulationelectric signal that modulates the equivalent refractive index issupplied to the first phase adjusting region 42 from a power source 76via a p-side electrode 56 of the first phase adjusting region, and thephase of an optical pulse passing through this region is modulated. Amodulation electric signal that modulates the equivalent refractiveindex is supplied to the second phase adjusting region 48 from a powersource 82 via a p-side electrode 62 of the second phase adjustingregion, and the phase of an optical pulse passing through this region ismodulated.

Sampled gratings 40G and 50G, that are formed at the first sampledgrating region 40 and the second sampled grating region 50 respectively,are structured such that a short-period grating is incorporated into oneperiod of a long-period grating, and are gratings having a double periodstructure of a long period and a short period. The first opticalintensity modulating region 44 and the second optical intensitymodulating region 52 have the function of modulating the opticalintensity. Further, the first optical intensity modulating region 44,the gain region 46, the first phase adjusting region 42 and the secondphase adjusting region 48 are disposed in series between the firstsampled grating region 40 and the second sampled grating region 50. Thesecond optical intensity modulating region 52 is outside of the regionthat is sandwiched between the first sampled grating region 40 and thesecond sampled grating region 50, and is disposed in series adjacent toeither one of the first sampled grating region 40 and the second sampledgrating region 50.

The Bragg reflection semiconductor laser 30 is mode-lock-operated by thelight transmittance of the first optical intensity modulating region 44being modulated by the first electric modulation signal 73, and can bemade to output a CS optical pulse train.

Further, the Bragg reflection semiconductor laser 30 can vary thewavelength of the oscillation light by changing the equivalentrefractive indices of the sampled grating regions, the first phaseadjusting region 42 and the second phase adjusting region 48. Moreover,the Bragg reflection semiconductor laser 30 can control the duty ratioof the optical pulses structuring the CS optical pulse train bymodulating the light transmittance of the second optical intensitymodulating region 52 by the second electric modulation signal 93.

The relationships of correspondence between the structural elements ofthe first CS optical pulse train generating device and the second CSoptical pulse train generating device are as follows. The two-mode beatlight source 18 of the first CS optical pulse train generating deviceshown in FIG. 1 corresponds to the portions other than the secondoptical intensity modulating region 52 of the Bragg reflectionsemiconductor laser 30 of the second CS optical pulse train generatingdevice. Further, the optical intensity modulator 20 of the first CSoptical pulse train generating device corresponds to the second opticalintensity modulating region 52 of the second CS optical pulse traingenerating device.

The structures and the functions of the first electric modulation signalgenerator 70 and the second electric modulation signal generator 90,that generate and output the first electric modulation signal 73 and thesecond electric modulation signal 93, are similar to the first CSoptical pulse train generating device. Therefore, repeat descriptionwill be omitted.

The Bragg reflection semiconductor laser 30 may be structured so as tohave only one of the first phase adjusting region 42 and the secondphase adjusting region 48. Further, the order of arrangement of thefirst phase adjusting region 42, the first optical intensity modulatingregion 44, the gain region 46, and the second phase adjusting region 48is not limited to the order shown in FIG. 9. For example, they may bearranged in the order of the first optical intensity modulating region44, the first phase adjusting region 42, the gain region 46, and thesecond phase adjusting region 48.

It is suitable that the band gap wavelength of the semiconductormaterial structuring the optical waveguides existing at the respectiveregions of the Bragg reflection semiconductor laser 30 be set asfollows. Namely, it is suitable that the band gap wavelengths of thesampled grating regions, the first phase adjusting region 42 and thesecond phase adjusting region 48 be set to the short wavelength sidewith respect to the laser oscillation wavelength that is determined bythe optical gain characteristic of the gain region 46 and the Braggreflecting characteristic at the sampled grating regions, and betransparent regions with respect to the laser oscillation light.

For example, in a case in which the Bragg reflection semiconductor laser30 is structured by an InP type semiconductor, the wavelength of thelaser oscillation light is in a vicinity of 1.55 μm. In this case, it issuitable that the optical waveguides existing at the sampled gratingregions, the first phase adjusting region 42 and the second phaseadjusting region 48 be structured by InGaAsP bulk crystal structures orquantum well structures of band gap wavelengths in the range of 1.1 μmto 1.3 μm.

On the other hand, it is suitable that the first optical intensitymodulating region 44 and the second optical intensity modulating region52 be set at a band gap composition which is a short wavelength withrespect to the laser oscillation wavelength, to the extent that, at thetime when the band gap wavelength shifts toward the long wavelength sidedue to inverse bias voltage that is applied to the region, the band endpartially overlaps the laser oscillation wavelength. In a case in whichthe wavelength of the laser oscillation light is in the vicinity of 1.55μm, it is suitable that the first optical intensity modulating region 44and the second optical intensity modulating region 52 be InGaAsP bulkcrystal structures or quantum well structures of band gap wavelengths of1.45 μm to 1.5 μm.

The structures and functions of the sampled gratings 40G and 50G, thatstructure the first sampled grating region 40 and the second sampledgrating region 50 respectively, will be described with reference to FIG.10A and FIG. 10B.

FIG. 10A is a drawing showing the equivalent refractive indexdistribution of the sampled gratings 40G and 50G along the lengthwisedirection of the optical waveguide that is the direction of propagationof the laser light. The equivalent refractive index can be varied byvarying the geometric thickness of the optical waveguide. The changedamount of the change in the geometric thickness of the opticalwaveguide, i.e., the state of change of the equivalent refractive index,is shown schematically so as to be marked on an arbitrary scale in thevertical axis direction of FIG. 10A.

FIG. 10B is a drawing showing the Bragg reflectance spectrum that is thereflection characteristic in accordance with the Bragg reflectingstructure, in a case in which the period structure of the equivalentrefractive index is Λ. The wavelength and the intensity of the reflectedlight are marked on arbitrary scales on the horizontal axis and thevertical axis of FIG. 10B, respectively.

The sampled gratings 40G and 50G, that structure the first sampledgrating region 40 and the second sampled grating region 50 respectively,are formed in accordance with the equivalent refractive indexdistribution shown in FIG. 10A (the distribution of the changes in thegeometric thickness of the optical waveguide). Namely, the sampledgratings 40G and 50G have a structure in which the portions wherediffraction grating of a uniform period Λ1 is formed and portions whereit is not formed are arranged periodically with respect to the directionof propagation of the laser light.

Hereinafter, the portions where the diffraction grating of the uniformperiod Λ1 is formed are called “mark portions”, and the portions wherethe diffraction grating is not formed are called “space portions”.Further, a sum Λ2 of the length of the mark portion and the length ofthe space portion is called a “sampling period”. Further, the period Λ1is called the “grating pitch Λ1”. The same holds for the second sampledgrating region 50 as well. Namely, the grating pitch of the secondsampled grating region 50 is Λ3, and the sampling period is Λ4.

Regularity is not particularly required between the grating pitch Λ1 andthe sampling period Λ2 of the first sampled grating region 40, and thegrating pitch Λ3 and the sampling period Λ4 of the second sampledgrating region 50. However, when considering the structure of the Braggreflection semiconductor laser 30, the structure that is easiest tomanufacture is as follows.

The optical waveguide layer 36, the n-side cladding layer 34, and thep-side cladding layer 38 of the first sampled grating region 40 and thesecond sampled grating region 50 are structured so as to have the samecomposition and the same optical waveguide widths. Then, by using alaser interference exposure method, the grating pitches Λ1 and Λ3 of thesampled gratings 40G and 50G respectively are structured to be equal.However, the sampling periods Λ2 and Λ4 of the sampled gratings 40G and50G respectively must be set to separate values for the followingreason.

The Bragg reflection semiconductor laser 30 generates two-mode beatlight due to laser light circling through the first phase adjustingregion 42, the first optical intensity modulating region 44, the gainregion 46 and the second phase adjusting region 48 that are disposed ata region between optical resonators. The region between the opticalresonators is formed by reflecting mirrors that are realized by thefirst sampled grating region 40 and the second sampled grating region50. In order to make the Bragg reflection semiconductor laser 30manifest mode-locking operation of the repetition frequency Δf, theoptical lengths of the respective regions of the first phase adjustingregion 42, the first optical intensity modulating region 44, the gainregion 46 and the second phase adjusting region 48 are adjusted suchthat the aforementioned optical resonator circling frequency of theBragg reflection semiconductor laser 30 becomes a frequencyapproximating Δf.

The optical resonator circling frequency being a frequency that“approximates” Δf means that the difference between an integer multipleof the optical resonator circling frequency of the Bragg reflectionsemiconductor laser 30, and the repetition frequency of the two-modebeat light that is the laser oscillation light generated at the regionbetween the optical resonators, is small to the extent that thefrequency pulling-in that is needed for causing mode-locking operationarises. Further, the “optical resonator circling frequency” is thereciprocal of the time required for the optical pulse to circle throughone time the optical resonator structured by the first sampled gratingregion 40 and the second sampled grating region 50.

A boundary face R between the second optical intensity modulating region52 and the exterior is subjected to a non-reflective coating processing.This is in order to prevent instability of the mode-locking operationdue to the threshold gain of the gain region 46 giving rise tofluctuations due to the light, that is reflected at the boundary face Rbetween the second optical intensity modulating region 52 and theexterior, becoming mixed-in within the optical resonator.

(Operation)

When current that is greater than or equal to the laser oscillationthreshold value is injected into the gain region 46 and the firstelectric modulation signal 73 is supplied from the first electricmodulation signal generator 70 to the first optical intensity modulatingregion 44, the mode-locking operation is manifested. Then, an opticalpulse train, that is synchronous with the first electric modulationsignal 73 and whose repetition frequency is Δf, is generated within theoptical resonator of the Bragg reflection semiconductor laser 30. Aswill be described later, the generated optical pulse train is two-modebeat light. The generated optical pulse train is inputted to the secondoptical intensity modulating region 52. From the second opticalintensity modulating region 52, the CS optical pulse trainpasses-through the boundary face R between the second optical intensitymodulating region 52 and the exterior, and is outputted to the exterior.

In the Bragg reflection semiconductor laser 30 shown in FIG. 9, insteadof providing the second optical intensity modulating region 52 adjacentto the second sampled grating region 50, the second optical intensitymodulating region 52 may be provided adjacent to the first sampledgrating region 40. At the optical resonator of the Bragg reflectionsemiconductor laser 30, two-mode beat light is generated and isoutputted to the exterior of the optical resonator. Namely, the placewhere the second optical intensity modulating region 52, that carriesout optical intensity modulation of the two-mode beat light, is providedmay be at either the left or the right of the optical resonator,provided that it is at the exterior of the optical resonator of theBragg reflection semiconductor laser 30.

The optical spectrum structure of the CS optical pulse train, thatpasses through the boundary face R between the exterior and the secondoptical intensity modulating region 52 of the Bragg reflectionsemiconductor laser 30 and is outputted to the exterior, will bedescribed. To this end, first, Bragg reflectance in a sampled gratingregion will be considered.

The reflection characteristic of a sampled grating having the equivalentrefractive index distribution structure shown in FIG. 10A is discussedin a thesis (V. Jayaraman, Z-M. Chuang, and L. A. Coldren, “Theory,Design, and Performance of Extended Tuning Range Semiconductor Laserswith Sampled Gratings”, IEEE Journal of Quantum Electronics, vol. 29,No. 6, pp. 1824-1834, 1993).

According to the above thesis by V. Jayaraman et al., the Braggreflection characteristic of a sampled grating having the equivalentrefractive index distribution structure shown in FIG. 10A is generally aBragg reflectance spectrum structure having multi-peaked reflectancepeaks as shown in FIG. 10B. This Bragg reflectance spectrum isstructured by a tanh-type (hyperbolic tangent type) reflection spectrumcomponent having the peak of reflectance at the wavelength provided byλBragg=2nΛ1, and plural tanh-type reflection spectrum components havingauxiliary peaks (maxima) at wavelengths that are separated from the peakreflection wavelength by integer multiples of λBragg2/(2nΛ2). Here, n isthe average value of the equivalent refractive index of the sampledgrating region.

The oscillation wavelength of a sampled grating type DBR laser isstructured to have, at both ends of the optical resonator, sampledgratings having the equivalent refractive index distribution structureshown in FIG. 10B. The oscillation wavelength of a sampled grating typeDBR laser is determined by the wavelength at which the value of theproduct of the Bragg reflectances of the sampled gratings at the bothends structuring the optical resonator becomes a maximum. If the maximuminterval λBragg2/(2nΛ2) of the Bragg reflectances of the sampledgratings at the both ends of the optical resonator differs and amechanism that can vary the equivalent refractive index n is formed, thelaser oscillation wavelength can be controlled by adjusting theequivalent refractive index n of the sampled gratings at both ends or atone end.

The phenomenon of the laser oscillation wavelength changing by adjustingthe equivalent refractive index n of the sampled gratings in this way iscalled the Vernier effect. The above thesis of V. Jayaraman et al.states that, by appropriately selecting the grating pitch, the samplingperiod, the ratio of the length of the space portion to the markportion, and the like of the sampled gratings structuring the opticalresonator, a wavelength variable characteristic of a wide range over awidth of 100 nm is obtained.

As described above, by structuring the optical resonator of the Braggreflection semiconductor laser 30 by sampled grating regions, theoscillation wavelength in the mode-locking operation can be varied overa wide range. Due thereto, the wavelength of the oscillation light thatis generated within the optical resonator can be varied within thisrange. Namely, the wavelength of the CS optical pulse train, that isoutputted from the boundary face R between the exterior and the secondoptical intensity modulating region 52 of the Bragg reflectionsemiconductor laser 30, can be varied over a wide range.

The principles of operation that make it possible to vary, over a widerange, the wavelength of the two-mode beat light that is generatedwithin the optical resonator of the Bragg reflection semiconductor laser30, will be described in detail with reference to FIG. 11A through FIG.11D and FIG. 12A through FIG. 12D.

FIG. 11A through FIG. 11D are drawings provided for explaining themechanism by which the wavelength of the two-mode beat light that isgenerated within the optical resonator of the Bragg reflectionsemiconductor laser 30, is determined. Further, FIG. 11A through FIG.11D are drawings showing a case in which the wavelengths of the mainpeaks of the Bragg reflectance spectra of the first sampled gratingregion 40 and the second sampled grating region 50 are the same. FIG.11A is a drawing showing the Bragg reflectance spectrum of the firstsampled grating region 40. FIG. 11B is a drawing showing the Braggreflectance spectrum of the second sampled grating region 50. FIG. 11Cis a drawing showing the product of both spectra of the Braggreflectances of the first and second sampled grating regions. FIG. 11Dis a drawing showing the longitudinal mode spectrum of the opticalresonator of the Bragg reflection semiconductor laser 30. In FIG. 11Athrough FIG. 11D, wavelength and optical intensity are marked on thehorizontal axis and on the vertical axis, respectively, on arbitraryscales.

First, as initial conditions, as shown in FIG. 11A through FIG. 11D, theBragg reflectance spectra of the first sampled grating region 40 and thesecond sampled grating region 50 have the same Bragg reflectionwavelengths, and the intervals between the maximum positions (peakwavelengths) of the Bragg reflectances are values that differ from oneanother.

A sampled grating region that satisfies such conditions can befabricated by a usual DBR laser manufacturing process. Namely, the markportions of the sampled grating regions are masked on the top surfacesof the optical waveguides 36 that have the same compositions and thesame widths and thicknesses, and grating structures are formed at theunmasked space portions by a usual laser interference exposure method soas to be the same grating pitch, i.e., such that Λ1=Λ3. Next, theaforementioned mask is removed, and the p-side cladding layer 38 of thesame composition is formed on the first sampled grating region 40 andthe second sampled grating region 50.

In order to change the interval λBragg2/(2nΛ2) between the maximumpositions (peak wavelengths) of the Bragg reflectance spectra of thefirst sampled grating region 40 and the second sampled grating region50, it suffices to change the interval of one or both of the markportions or space portions. The periodic equivalent refractive indexstructures of the sampled grating regions can be fabricated by theaforementioned laser interference exposure method, or by an electronbeam exposure method as well.

The maximum position (peak wavelength) intervals of the Braggreflectance spectra of the first sampled grating region 40 and thesecond sampled grating region 50 are values that are different from oneanother. Therefore, as shown in FIG. 11C, the main peak of thewavelength spectrum provided by the product of the both appears at aposition that is equal to the Bragg reflection wavelength. Namely, thisis a substantially single-peaked spectrum structure. The laseroscillation of the Bragg reflection semiconductor laser 30 arises in theresonator mode that approximates the main peak of this single-peakedspectrum.

Here, the following are set in order for the frequency spectrum band,that is obtained by converting the above-described wavelength spectrumof the sampled grating into frequency, to be the same extent as therepetition frequency Δf of the oscillation light generated within theoptical resonator of the Bragg reflection semiconductor laser 30: thecoupling coefficient of the first sampled grating region 40 and thesecond sampled grating region 50; the sampling period; the ratio of thelengths of the mark portion and the space portion of the sampling; andthe sampling number that is defined as the number of repetitions of themark portion and the space portion. In this case, as shown in FIG. 11D,there are at the most about two modulation sidebands of the frequencyinterval Δf of the laser oscillation light generated within the opticalresonator of the Bragg reflection semiconductor laser 30, whichmodulation sidebands arise within the frequency band. These twosidebands (longitudinal mode spectrum components) exist at symmetricalpositions with respect to the main peak position shown in FIG. 11D.

Namely, the number of longitudinal modes of the oscillation of the Braggreflection semiconductor laser 30 is limited to about 2 at the most.Further, if the longitudinal mode positions of the longitudinal modespectrum of the Bragg reflection semiconductor laser 30 are adjusted soas to become symmetrical with respect to the peak wavelength position ofthe wavelength spectrum of the sampled grating, the two sidebands thatare adjacent to the peak wavelength of the wavelength spectrum of thesampled grating satisfy laser oscillation conditions that are equivalentto one another.

The wavelength spectrum of the oscillation light, that is generatedwithin the optical resonator of the Bragg reflection semiconductor laser30 in this state, is structured from two modulation sidebands havingequal peak intensities. At this time, the optical intensity timewaveform of the generated oscillation light is a sine wave, and as aresult, this oscillation light is two-mode beat light. Accordingly, inaccordance with the second CS optical pulse train generating device, thetwo-mode beat light generated within the optical resonator of the Braggreflection semiconductor laser 30 is, in the same way as the opticalintensity modulator 20 of the first CS optical pulse train generatingdevice, optical-intensity-modulated by the second optical intensitymodulating region 52, and is generated and outputted as a CS opticalpulse train. In this way, in accordance with the second CS optical pulsetrain generating device, the duty ratio of the CS optical pulse trainthat is generated and outputted can be varied over a wide range.

A plasma effect is manifested by carrying out current injection at thefirst phase adjusting region 42 and the second phase adjusting region48, and the equivalent refractive index of the first phase adjustingregion 42 and the second phase adjusting region 48 can thereby beadjusted. Accordingly, the adjusting of the longitudinal mode positionof the longitudinal mode spectrum of the optical resonator of the Braggreflection semiconductor laser 30 is realized by adjusting the currentvalue injected to at least one of the first phase adjusting region 42and the second phase adjusting region 48.

Further, the Pockels effect is manifested by applying inverse biasvoltage to the first phase adjusting region 42 and the second phaseadjusting region 48, and the equivalent refractive index of the firstphase adjusting region 42 and the second phase adjusting region 48 canthereby be adjusted. Accordingly, in this case, the adjusting of thelongitudinal mode position of the longitudinal mode spectrum of theoptical resonator of the Bragg reflection semiconductor laser 30 isrealized by adjusting the inverse bias voltage that is applied to atleast one of the first phase adjusting region 42 and the second phaseadjusting region 48.

Next, a case in which the Bragg reflection wavelengths of either one orboth regions of the Bragg reflectance spectra of the first sampledgrating region 40 and the second sampled grating region 50 do notcoincide, will be studied. Such a state is realized by carrying outcurrent injection or applying voltage by the power source 74 and thepower source 84 respectively to either one or both regions of the Braggreflectance spectra of the first sampled grating region 40 and thesecond sampled grating region 50.

The plasma effect can be manifested by carrying out current injectioninto the first sampled grating region 40 and the second sampled gratingregion 50. Due to the plasma effect, the equivalent refractive indicesof both of the sampled grating regions are changed, and the Braggreflection wavelength can be changed. Further, the Pockels effect can bemanifested by applying voltage to the first sampled grating region 40and the second sampled grating region 50. Also due thereto, theequivalent refractive indices of both of the sampled grating regions arechanged, and the Bragg reflection wavelength can be changed. In eithercase, the Bragg reflection wavelength can be changed, and therefore, astate in which the Bragg reflection wavelengths of either one or bothregions of the Bragg reflectance spectra of the first sampled gratingregion 40 and the second sampled grating region 50 do not coincide canbe realized.

FIG. 12A through FIG. 12D are drawings that are provided for explainingthe mechanism by which the wavelength of the two-mode beat light, thatis generated within the optical resonator of the Bragg reflectionsemiconductor laser 30, is determined. Further, FIG. 12A through FIG.12D are drawings showing a case in which the auxiliary peak of the Braggreflectance spectrum of the first sampled grating region 40 and theauxiliary peak of the Bragg reflectance spectrum of the second sampledgrating region 50 are different. Namely, FIG. 12A through FIG. 12D aredrawings showing a case in which the Bragg reflection wavelengths aredifferent. FIG. 12A is a drawing showing the Bragg reflectance spectrumof the first sampled grating region 40. FIG. 12B is a drawing showingthe Bragg reflectance spectrum of the second sampled grating region 50.FIG. 12C is a drawing showing the product of the both spectra of theBragg reflectances of the first and second sampled grating regions. FIG.12D is a drawing showing the longitudinal mode spectrum of the opticalresonator of the Bragg reflection semiconductor laser 30. In FIG. 12Athrough FIG. 12D, wavelength and optical intensity are marked onarbitrary scales on the horizontal axis and the vertical axis,respectively.

The main peak of the Bragg reflectance spectrum shown in FIG. 12B isshifted, with respect to the Bragg reflectance spectrum shown in FIG.12A, by carrying out current injection or by applying voltage by thepower source 74 and the power source 84 respectively to either one orboth of the first sampled grating region 40 and the second sampledgrating region 50.

When adjustment is carried out such that the auxiliary peak of the Braggreflectance spectrum of the first sampled grating region 40 and theauxiliary peak of the Bragg reflectance spectrum of the second sampledgrating region 50 coincide, the product of the Bragg reflectance spectraof the both is a substantially single-peaked spectrum structure as shownin FIG. 12C. At the optical resonator of the Bragg reflectionsemiconductor laser 30, laser oscillation arises in a resonator modethat approximates the main peak of this single-peaked spectrum.

Here, in the same way as the case explained with reference to theabove-described FIG. 11A through FIG. 11D, the following are set inorder for the frequency spectrum band, that is obtained by convertingthe wavelength spectrum of the sampled grating into frequency, to be thesame extent as the repetition frequency Δf of the oscillation lightgenerated within the optical resonator of the Bragg reflectionsemiconductor laser 30: the coupling coefficient of the first sampledgrating region 40 and the second sampled grating region 50; the samplingperiod; the ratio of the lengths of the mark portion and the spaceportion of the sampling; and the sampling number that is defined as thenumber of repetitions of the mark portion and the space portion.

In this case, in the same way as the case explained with reference toabove-described FIG. 11A through FIG. 11D, within the optical resonatorof the Bragg reflection semiconductor laser 30, two-mode beat lightwhose optical intensity time waveform is a sine wave is generated. Inthis case, as shown in FIG. 12D, within the frequency band, there are atthe most about two modulation sidebands (longitudinal mode spectrumcomponents) of the frequency interval Δf of the oscillation lightgenerated within the optical resonator of the Bragg reflectionsemiconductor laser 30. These two sidebands exist at symmetricalpositions with respect to the main peak position shown in FIG. 12D.

As a result, in accordance with the second CS optical pulse traingenerating device, the two-mode beat light, that is generated within theoptical resonator formed by the first sampled grating region 40 and thesecond sampled grating region 50 of the Bragg reflection semiconductorlaser 30, is optical-intensity-modulated by the second optical intensitymodulating region 52, and is generated and outputted as a CS opticalpulse train. The optical intensity modulation and the generating of theCS optical pulse train by the second optical intensity modulating region52 are similar to the optical intensity modulator 20 of the first CSoptical pulse train generating device. In this way, in accordance withthe second CS optical pulse train generating device, the duty ratio ofthe CS optical pulse train that is generated and outputted can be variedwithin a wide range.

In the following description, what should be called the “opticalresonator that is formed by the first sampled grating region 40 and thesecond sampled grating region 50” will, upon occasion, simply be calledthe “optical resonator”.

Next, at the second CS optical pulse train generating device, thestructure of the Bragg reflection semiconductor laser 30 is set suchthat the first optical intensity modulating region 44 is disposed at aposition that is the center of the optical resonator, and the 2Nth orderhigher harmonic mode-locking operation is manifested at the time whenthe first electric modulation signal 73 whose frequency is Δf isinputted to the first optical intensity modulating region 44. Theeffects obtained due thereto will be described hereinafter. Here, N isan integer of greater than or equal to 1.

The position of the center of the optical resonator is within theoptical resonator, and indicates a position at which the time, until theoptical pulse that has passed through the first optical intensitymodulating region 44 is Bragg-reflected at the first sampled gratingregion 40 and returns to the first optical intensity modulating region44, and the time, until the optical pulse that has passed through thefirst optical intensity modulating region 44 is Bragg-reflected at thesecond sampled grating region 50 and returns to the first opticalintensity modulating region 50, become equal.

Namely, in a case in which the structure of the Bragg reflectionsemiconductor laser 30 is set such that the above-described firstoptical intensity modulating region 44 is disposed at the position ofthe center of the optical resonator and 2Nth order higher harmonicmode-locking operation is manifested, the reciprocal of the opticalresonator circling frequency, that is the time required for an opticalpulse to circle through the optical resonator one time, substantiallycoincides with 2N/Δf. In other words, the optical resonator circlingfrequency approximates 1/(2N) of the repetition frequency Δf of thefirst electric modulation signal 73.

This condition corresponds to both the above-described time, until theoptical pulse that has passed through the first optical intensitymodulating region 44 is Bragg-reflected at the first sampled gratingregion 40 and returns to the first optical intensity modulating region44, and the above-described time, until the optical pulse that haspassed through the first optical intensity modulating region 44 isBragg-reflected at the second sampled grating region 50 and returns tothe first optical intensity modulating region 44, coinciding with N/Δf.

Here, the optical resonator circling frequency “approximating” therepetition frequency of the first electric modulation signal 73 meansthat the difference between a positive integer multiple of the opticalresonator circling frequency and the repetition frequency of the firstelectric modulation signal 73 is small to the extent that the frequencypulling-in needed for mode-locking operation arises.

The time required for an optical pulse to circle through the opticalresonator once is equal to the value obtained by dividing a given valueby light speed within a vacuum, where the given value is obtained bydoubling the sum obtained by adding the penetration lengths of the firstsampled grating region 40 and the second sampled grating region 50 tothe optical lengths of the respective regions of the first phaseadjusting region 42, the first optical intensity modulating region 44,the gain region 46 and the second phase adjusting region 48. Here,“optical length” is a value obtained by multiplying the geometric length(also called the physical length) by the equivalent refractive index.Further, “penetration length of the sampled grating region” means theequivalent optical length that is obtained by adding the reduction inthe effective region length due to the Bragg reflection of the sampledgrating. Namely, while the optical pulse inputted to the sampled gratingregion is Bragg-reflected, the optical intensity thereof is reduced andthe optical pulse advances through the sampled grating region. Thedistance from the incident end of the sampled grating region to theposition where the optical intensity of the optical pulse becomes anintensity of 1/e of the optical intensity at the incident end is calledthe “penetration length of the sampled grating region”. Here, e is thebase of the natural logarithm.

Mode-locking operation of the fundamental order is realized at theoptical resonator of the Bragg reflection semiconductor laser 30 of thesecond CS optical pulse train generating device. The “mode-lockingoperation of the fundamental order” means that the optical resonatorcircling frequency approximates the frequency Δf of the first electricmodulation signal 73.

In the second CS optical pulse train generating device, in order toincrease the repetition frequency Δf of the CS optical pulse train to beoutputted, it is necessary to increase the optical resonator circlingfrequency. To this end, it suffices to shorten the physical lengths ofthe respective regions that are the first phase adjusting region 42, thefirst optical intensity modulating region 44, the gain region 46 and thesecond phase adjusting region 48 that structure the optical resonator,and the penetration lengths of the first sampled grating region 40 andthe second sampled grating region 50.

However, there is a limit to making short the physical lengths of therespective regions structuring the optical resonator, and thepenetration lengths of the sampled grating regions. For example, if thephysical length of the gain region 46 is made to be too short, the gainneeded for laser oscillation cannot be obtained. Further, if thephysical length of the first optical intensity modulating region 44 ismade to be too short, optical intensity modulation cannot be carried outto the depth of the extent needed for mode-locking operation.

Further, there are also limits to making the physical lengths of thesampled grating regions short. First, in order to obtain the effect ofthe sampled grating, quite a large number of pairs, which pair is themark portion and the space portion of the grating, must be ensured. If alarge number of pairs is ensured, the sampling period must be made to beshort of necessity. If the sampling period is made to be too short, thepeak interval (λBragg2/(2nΛ2)) of the Bragg reflectance spectrum of thesampled grating region becomes an extremely large value, and wavelengthvariable operation that is continuous cannot be realized.

Further, if the physical length of the sampled grating region is tooshort, it is difficult to make the Bragg reflectance large. As a result,the laser oscillation threshold value increases, and the Braggreflectance spectrum band widens, and it is thereby difficult to obtainselective two-mode oscillation.

In contrast with this situation, the Bragg reflection semiconductorlaser 30 of the second CS optical pulse train generating device is setsuch that the first optical intensity modulating region 44 is disposedat the position of the center of the optical resonator, and 2Nth orderhigher harmonic mode-locking operation is manifested at the time whenthe first electric modulation signal 73 whose frequency is Δf isinputted to the first optical intensity modulating region 44. Thefollowing effects are thereby obtained.

Namely, by setting the device to such conditions, the reciprocal of theoptical resonator circling frequency can be made to substantiallycoincide with 2N/Δf. This means that the optical length of the opticalresonator becomes 2N times that of the case of causing basicmode-locking operation to be manifested. Accordingly, by setting thedevice to the conditions that the first optical intensity modulatingregion 44 is disposed at the position of the center of the opticalresonator and 2Nth order higher harmonic mode-locking operation ismanifested, there is no longer the need to make short the physicallengths of the respective regions structuring the optical resonator andthe penetration lengths of the sampled grating regions when increasingthe repetition frequency Δf of the CS optical pulse train that isgenerated and outputted at the second CS optical pulse train generatingdevice.

In the state in which the 2Nth order higher harmonic mode-lockingoperation is being manifested, during the time that an optical pulsecircles through the optical resonator once, the optical pulse passesthrough the first optical intensity modulating region 44 twice andreceives optical intensity modulation. In order to maintain the 2Nthorder higher harmonic mode-locking operation temporally stable, the twooptical intensity modulations that the optical pulse receives at thefirst optical intensity modulating region 44 must be realized to anequal extent.

For example, if the relative relationship between the time that thelight transmittance of the first optical intensity modulating region 44becomes the minimum and the time that the peak of the optical pulsepasses through the first optical intensity modulating region 44 aredifferent in each of the aforementioned two optical intensitymodulations, an optical pulse having two separate characteristics isgrown. In this case, at the first optical intensity modulating region44, optical intensity modulation arises at the optical pulse that isgenerated within the optical resonator, and it is difficult to generateregular two-mode beat light whose peak intensities are uniform.

In order to avoid a situation in which it is difficult to generateregular two-mode beat light whose peak intensities are uniform withinthe optical resonator in this way, it suffices to dispose the firstoptical intensity modulating region 44 at the position of the center ofthe optical resonator. By doing so, the relative relationship betweenthe time that the light transmittance of the first optical intensitymodulating region 44 becomes the minimum and the time that the peak ofthe optical pulse passes through the first optical intensity modulatingregion 44, coincides in each of the aforementioned two optical intensitymodulations. Therefore, an optical pulse having the same characteristicis grown within the optical resonator, and a regular CS optical pulsetrain whose peak intensities are uniform is generated.

It suffices to set the sampled grating regions as follows. Namely, itsuffices to set the following so that the peak frequency of thesingle-peaked Bragg reflectance spectrum, that is provided by theproduct of the Bragg reflectance spectrum of the first sampled gratingregion 40 and the Bragg reflectance spectrum of the second sampledgrating region 50, approximates the repetition frequency Δf of thetwo-mode beat light generated within the optical resonator: the couplingcoefficient of the sampled grating regions; the sampling period; theratio of the lengths of the mark portion and the space portion of thesampling; and the sampling number that is defined as the number ofrepetitions of the mark portion and the space portion.

Here, the peak frequency of the single-peaked Bragg reflectance spectrum“approximating” the repetition frequency of the two-mode beat lightgenerated within the optical resonator means that the difference betweenthe peak frequency of the single-peaked Bragg reflectance spectrum andthe repetition frequency of the two-mode beat light generated within theoptical resonator, is small to the extent that the frequency pulling-inthat is needed for causing mode-locking operation arises.

As described above, the device is set such that the first opticalintensity modulating region 44 is disposed at a position that is thecenter of the optical resonator, and the 2Nth order higher harmonicmode-locking operation is manifested at the time when the first electricmodulation signal 73 whose frequency is Δf is inputted to the firstoptical intensity modulating region 44. Due thereto, the followingeffects are obtained. Namely, even in cases in which the repetitionfrequency Δf is high, there is no need to make the optical resonatorlength extremely short. As a result, the laser oscillation thresholdvalue is reduced, and a CS optical pulse train, that has sufficientintensity and a high repetition frequency, can be generated andoutputted.

The above-described exemplary embodiment assumes a case in which thewavelength of the CS optical pulse train to be generated is the band of1.5 μm. However, the second CS optical pulse train generating device ofthe invention can be designed as a device that generates a CS opticalpulse train even at other than this frequency band. For example,structuring the second CS optical pulse train generating device of theinvention as a device that generates a CS optical pulse train of awavelength of the band of 0.8 μm is realized by making the semiconductormaterial that structures the Bragg reflection semiconductor laser 30 bea GaAs type semiconductor. Namely, the second CS optical pulse traingenerating device is a device that can be realized without being limitedin theory by the wavelength of the CS optical pulse train to begenerated.

Further, in the above-described exemplary embodiment, in the opticalresonator of the Bragg reflection semiconductor laser 30, themode-locking operation is manifested by supplying the first electricmodulation signal 73 to the first optical intensity modulating region 44from the exterior. Namely, active mode synchronization is manifested atthe optical resonator. In contrast, the second CS optical pulse traingenerating device can similarly be realized by realizing passivemode-locking operation that causes mode-locking operation bycontrolling, from the exterior, the first optical intensity modulatingregion 44 to function as a saturatable absorber.

However, if the second CS optical pulse train generating device isrealized on the basis of passive mode-locking operation, in order togenerate an optical pulse train (two-mode beat light) that is generatedwithin the optical resonator synchronously with the first electricmodulation signal 73, the first electric modulation signal 73 thatincludes, as a bias component, a DC component needed for causing thefirst optical intensity modulating region 44 to function as asaturatable absorber, must be supplied from the exterior. Namely, inthis case, two-mode beat light is generated within the optical resonatorby causing the first optical intensity modulating region 44 to functionas a saturatable absorber and realizing hybrid mode-locking operation.

1. A carrier-suppressed optical pulse train generating devicecomprising: a first electric modulation signal generator generating andoutputting a first electric modulation signal that is synchronous with aclock signal; a second electric modulation signal generator generatingand outputting a second electric modulation signal of a same frequencyas the first electric modulation signal and to which a phase differenceof δ radians is provided, where δ is a real number satisfying 0≦δ≦π; atwo-mode beat light source driven by the first electric modulationsignal, and generating and outputting two-mode beat light; and anoptical intensity modulator to which the two-mode beat light isinputted, and that optical-intensity-modulates the two-mode beat light,and generates and outputs a carrier-suppressed optical pulse trainhaving numerous longitudinal modes whose longitudinal mode spectra aregreater than 2, wherein light transmittance of the optical intensitymodulator is modulated by the second electric modulation signal.
 2. Thecarrier-suppressed optical pulse train generating device of claim 1,wherein a value ofδ is
 0. 3. The carrier-suppressed optical pulse traingenerating device of claim 1, wherein a value of δ is π.
 4. Thecarrier-suppressed optical pulse train generating device of claim 1,wherein a value of δ is 0, and a bias value and a value of an intensityamplitude of the second electric modulation signal are set such that aminimum value of the light transmittance of the optical intensitymodulator is
 0. 5. The carrier-suppressed optical pulse train generatingdevice of claim 1, wherein a value of δ is π, and a bias value and avalue of an intensity amplitude of the second electric modulation signalare set such that an extinction ratio, that is defined as a ratio of amaximum value and a minimum value of the light transmittance of theoptical intensity modulator, is a maximum value of immediately beforeoccurrence of a splitting phenomenon that divides a peak of a singleoptical pulse structuring the carrier-suppressed optical pulse traininto a plurality of peaks.
 6. A carrier-suppressed optical pulse traingenerating device comprising: a first electric modulation signalgenerator generating and outputting a first electric modulation signalthat is synchronous with a clock signal; a second electric modulationsignal generator generating and outputting a second electric modulationsignal of a same frequency as the first electric modulation signal andto which a phase difference of δ radians is provided, where δ is a realnumber satisfying 0≦δ≦π; and a Bragg reflection semiconductor laser,wherein the Bragg reflection semiconductor laser comprises: first andsecond sampled grating regions at which are formed sampled gratings thatare structured such that a short-period grating is incorporated-inwithin one period of a long-period grating, and that have a doubleperiod structure of a long period and a short period; first and secondoptical intensity modulating regions having a function of modulatingoptical intensity; a gain region at which an inverted distribution isformed; and first and second phase adjusting regions at which anequivalent refractive index is variable, wherein a Bragg reflectionsemiconductor laser structure is formed by disposing, in series, thefirst optical intensity modulating region, the gain region and the firstand second phase adjusting regions, between the first sampled gratingregion and the second sampled grating region, the second opticalintensity modulating region is outside of a region sandwiched by thefirst sampled grating region and the second sampled grating region, andis structured by being disposed in series and adjacent to either one ofthe first sampled grating region and the second sampled grating region,a wavelength of oscillation light of a Bragg reflection semiconductorlaser structural portion can be varied by changing equivalent refractiveindices of the first and second sampled grating regions and the firstand second phase adjusting regions, the laser is mode-lock-operated bymodulating light transmittance of the first optical intensity modulatingregion by the first electric modulation signal, and can be made tooutput a carrier-suppressed optical pulse train, and a duty ratio of anoptical pulse structuring the carrier-suppressed optical pulse train canbe controlled by modulating light transmittance of the second opticalintensity modulating region by the second electric modulation signal. 7.The carrier-suppressed optical pulse train generating device of claim 6,wherein the first optical intensity modulating region is within anoptical resonator that is formed by the first sampled grating region andthe second sampled grating region, and is disposed at a position that isa center of the optical resonator where both a time, until an opticalpulse that has passed through the first optical intensity modulatingregion is Bragg-reflected at the first sampled grating region andreturns to the first optical intensity modulating region, and a time,until an optical pulse that has passed through the first opticalintensity modulating region is Bragg-reflected at the second sampledgrating region and returns to the first optical intensity modulatingregion, are equal to N/Δf, where N is an integer of greater than orequal to 1 and Δf is a repetition frequency of an optical pulse of thecarrier-suppressed optical pulse train that is an optical pulse train.8. The carrier-suppressed optical pulse train generating device of claim6, wherein a value of δ is
 0. 9. The carrier-suppressed optical pulsetrain generating device of claim 6, wherein a value of δ is π.
 10. Thecarrier-suppressed optical pulse train generating device of claim 6,wherein a value of δ is 0, and a bias value and a value of an intensityamplitude of the second electric modulation signal are set such that aminimum value of light transmittance of the first and second opticalintensity modulating regions is
 0. 11. The carrier-suppressed opticalpulse train generating device of claim 6, wherein a value of δ is π, anda bias value and a value of an intensity amplitude of the secondelectric modulation signal are set such that an extinction ratio, thatis defined as a ratio of a maximum value and a minimum value of thelight transmittance of the first and second optical intensity modulatingregions, is a maximum value of immediately before occurrence of asplitting phenomenon that divides a peak of a single optical pulsestructuring the carrier-suppressed optical pulse train into a pluralityof peaks.
 12. A carrier-suppressed optical pulse train generating methodcomprising: a first electric modulation signal generating stepgenerating and outputting, by a first electric modulation signalgenerator, a first electric modulation signal that is synchronous with aclock signal; a second electric modulation signal generating stepgenerating and outputting, by a second electric modulation signalgenerator, a second electric modulation signal of a same frequency asthe first electric modulation signal and having a phase difference of δradians, where δ is a real number satisfying 0≦δ≦π; a two-mode beatlight generating step driving a two-mode beat light source by the firstelectric modulation signal, and generating and outputting two-mode beatlight that is synchronous with the clock signal; and an opticalintensity modulating step optical-intensity-modulating the two-mode beatlight by an optical intensity modulator that is driven by the secondelectric modulation signal, and generating and outputting acarrier-suppressed optical pulse train having numerous longitudinalmodes whose longitudinal mode spectra are greater than
 2. 13. Thecarrier-suppressed optical pulse train generating method of claim 12,wherein a value of δ is
 0. 14. The carrier-suppressed optical pulsetrain generating method of claim 12, wherein a value of δ is π.
 15. Thecarrier-suppressed optical pulse train generating method of claim 12,wherein a value of δ is 0, and a bias value and a value of an intensityamplitude of the second electric modulation signal are set such that aminimum value of light transmittance of the optical intensity modulatoris
 0. 16. The carrier-suppressed optical pulse train generating methodof claim 12, wherein a value of δ is π, and a bias value and a value ofan intensity amplitude of the second electric modulation signal are setsuch that an extinction ratio, that is defined as a ratio of a maximumvalue and a minimum value of light transmittance of the opticalintensity modulator, is a maximum value of immediately before occurrenceof a splitting phenomenon that divides a peak of a single optical pulsestructuring the carrier-suppressed optical pulse train into a pluralityof peaks.
 17. A carrier-suppressed optical pulse train generating methodusing a Bragg reflection semiconductor laser comprising: first andsecond sampled grating regions at which are formed sampled gratings thatare structured such that a short-period grating is incorporated-inwithin one period of a long-period grating, and that have a doubleperiod structure of a long period and a short period; first and secondoptical intensity modulating regions having a function of modulatingoptical intensity; a gain region at which an inverted distribution isformed; and first and second phase adjusting regions at which anequivalent refractive index is variable, where a Bragg reflectionsemiconductor laser structure is formed by disposing, in series, thefirst optical intensity modulating region, the gain region and the firstand second phase adjusting regions, between the first sampled gratingregion and the second sampled grating region, the second opticalintensity modulating region is outside of a region sandwiched by thefirst sampled grating region and the second sampled grating region, andis structured by being disposed in series and adjacent to either one ofthe first sampled grating region and the second sampled grating region,a wavelength of oscillation light of a Bragg reflection semiconductorlaser structural portion can be varied by changing equivalent refractiveindices of the first and second sampled grating regions and the firstand second phase adjusting regions, the laser is mode-lock-operated bymodulating light transmittance of the first optical intensity modulatingregion, and can be made to output a carrier-suppressed optical pulsetrain, and a duty ratio of an optical pulse structuring thecarrier-suppressed optical pulse train can be controlled by modulatinglight transmittance of the second optical intensity modulating region,the method comprising: a first electric modulation signal generatingstep generating and outputting, by a first electric modulation signalgenerator, a first electric modulation signal that is synchronous with aclock signal; a second electric modulation signal generating stepgenerating and outputting, by a second electric modulation signalgenerator, a second electric modulation signal of a same frequency asthe first electric modulation signal and having a phase difference of δradians; a wavelength adjusting step varying a wavelength of oscillationlight at a Bragg reflection semiconductor laser structural portion, bychanging equivalent refractive indices of the first and second sampledgrating regions and the first and second phase adjusting regions; amode-lock operating step causing mode-locking operation by modulatinglight transmittance of the first optical intensity modulating region bythe first electric modulation signal; and a duty ratio adjusting stepcontrolling a duty ratio of an optical pulse structuring acarrier-suppressed optical pulse train by modulating light transmittanceof the second optical intensity modulating region by the second electricmodulating signal.
 18. The carrier-suppressed optical pulse traingenerating method of claim 17, wherein the first optical intensitymodulating region of the Bragg reflection semiconductor laser portion iswithin an optical resonator that is formed by the first sampled gratingregion and the second sampled grating region, and is disposed at aposition that is a center of the optical resonator where both a time,until an optical pulse that has passed through the first opticalintensity modulating region is Bragg-reflected at the first sampledgrating region and returns to the first optical intensity modulatingregion, and a time, until an optical pulse that has passed through thefirst optical intensity modulating region is Bragg-reflected at thesecond sampled grating region and returns to the first optical intensitymodulating region, are equal to N/Δf, where N is an integer of greaterthan or equal to 1 and Δf is a repetition frequency of an optical pulseof the carrier-suppressed optical pulse train that is an optical pulsetrain.
 19. The carrier-suppressed optical pulse train generating methodof claim 17, wherein a value of δ is
 0. 20. The carrier-suppressedoptical pulse train generating method of claim 17, wherein a value of δis π.
 21. The carrier-suppressed optical pulse train generating methodof claim 17, wherein a value of δ is 0, and a bias value and a value ofan intensity amplitude of the second electric modulation signal are setsuch that a minimum value of light transmittance of the first and secondoptical intensity modulating regions is
 0. 22. The carrier-suppressedoptical pulse train generating method of claim 17, wherein a value of δis π, and a bias value and a value of an intensity amplitude of thesecond electric modulation signal are set such that an extinction ratio,that is defined as a ratio of a maximum value and a minimum value of thelight transmittance of the first and second optical intensity modulatingregions, is a maximum value of immediately before occurrence of asplitting phenomenon that divides a peak of a single optical pulsestructuring the carrier-suppressed optical pulse train into a pluralityof peaks.